I have Googled a lot and found a lot of solutions, but none of them give me the correct week number for the 2012-12-31. Even the example on MSDN (link) fails.
2012-12-31 is Monday, therefore it should be Week 1, but every method I tried gives me 53. Here are some of the methods, that I have tried:
From the MDSN Library:
DateTimeFormatInfo dfi = DateTimeFormatInfo.CurrentInfo;
Calendar cal = dfi.Calendar;
return cal.GetWeekOfYear(date, dfi.CalendarWeekRule, dfi.FirstDayOfWeek);
Solution 2:
return new GregorianCalendar(GregorianCalendarTypes.Localized).GetWeekOfYear(date, CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday);
Solution 3:
CultureInfo ciCurr = CultureInfo.CurrentCulture;
int weekNum = ciCurr.Calendar.GetWeekOfYear(dtPassed, CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday);
return weekNum;
Update
The following method actually returns 1 when date is 2012-12-31. In other words, my problem was that my methods were not following the ISO-8601 standard.
// This presumes that weeks start with Monday.
// Week 1 is the 1st week of the year with a Thursday in it.
public static int GetIso8601WeekOfYear(DateTime time)
{
// Seriously cheat. If its Monday, Tuesday or Wednesday, then it'll
// be the same week# as whatever Thursday, Friday or Saturday are,
// and we always get those right
DayOfWeek day = CultureInfo.InvariantCulture.Calendar.GetDayOfWeek(time);
if (day >= DayOfWeek.Monday && day <= DayOfWeek.Wednesday)
{
time = time.AddDays(3);
}
// Return the week of our adjusted day
return CultureInfo.InvariantCulture.Calendar.GetWeekOfYear(time, CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday);
}
As noted in this MSDN page there is a slight difference between ISO8601 week and .Net week numbering.
You can refer to this article in MSDN Blog for a better explanation: "ISO 8601 Week of Year format in Microsoft .Net"
Simply put, .Net allow weeks to be split across years while the ISO standard does not.
In the article there is also a simple function to get the correct ISO 8601 week number for the last week of the year.
Update The following method actually returns 1 for 2012-12-31 which is correct in ISO 8601 (e.g. Germany).
// This presumes that weeks start with Monday.
// Week 1 is the 1st week of the year with a Thursday in it.
public static int GetIso8601WeekOfYear(DateTime time)
{
// Seriously cheat. If its Monday, Tuesday or Wednesday, then it'll
// be the same week# as whatever Thursday, Friday or Saturday are,
// and we always get those right
DayOfWeek day = CultureInfo.InvariantCulture.Calendar.GetDayOfWeek(time);
if (day >= DayOfWeek.Monday && day <= DayOfWeek.Wednesday)
{
time = time.AddDays(3);
}
// Return the week of our adjusted day
return CultureInfo.InvariantCulture.Calendar.GetWeekOfYear(time, CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday);
}
Good news! A pull request adding System.Globalization.ISOWeek to .NET Core was just merged and is currently slated for the 3.0 release. Hopefully it will propagate to the other .NET platforms in a not-too-distant future.
The type has the following signature, which should cover most ISO week needs:
namespace System.Globalization
{
public static class ISOWeek
{
public static int GetWeekOfYear(DateTime date);
public static int GetWeeksInYear(int year);
public static int GetYear(DateTime date);
public static DateTime GetYearEnd(int year);
public static DateTime GetYearStart(int year);
public static DateTime ToDateTime(int year, int week, DayOfWeek dayOfWeek);
}
}
You can find the source code here.
UPDATE: These APIs have also been included in the 2.1 version of .NET Standard.
There can be more than 52 weeks in a year. Each year has 52 full weeks + 1 or +2 (leap year) days extra. They make up for a 53th week.
52 weeks * 7days = 364 days.
So for each year you have at least one an extra day. Two for leap years. Are these extra days counted as separate weeks of their own?
How many weeks there are really depends on the starting day of your week. Let's consider this for 2012.
US (Sunday -> Saturday): 52 weeks + one short 2 day week for 2012-12-30 & 2012-12-31. This results in a total of 53 weeks. Last two days of this year (Sunday + Monday) make up their own short week.
Check your current Culture's settings to see what it uses as the first day of the week.
As you see it's normal to get 53 as a result.
Europe (Monday -> Sunday): January 2dn (2012-1-2) is the first monday, so this is the first day of the first week. Ask the week number for the 1st of January and you'll get back 52 as it is considered part of 2011 last's week.
It's even possible to have a 54th week. Happens every 28 years when the 1st of January and the 31st of December are treated as separate weeks. It must be a leap year too.
For example, the year 2000 had 54 weeks. January 1st (sat) was the first one week day, and 31st December (sun) was the second one week day.
var d = new DateTime(2012, 12, 31);
CultureInfo cul = CultureInfo.CurrentCulture;
var firstDayWeek = cul.Calendar.GetWeekOfYear(
d,
CalendarWeekRule.FirstDay,
DayOfWeek.Monday);
int weekNum = cul.Calendar.GetWeekOfYear(
d,
CalendarWeekRule.FirstDay,
DayOfWeek.Monday);
int year = weekNum == 52 && d.Month == 1 ? d.Year - 1 : d.Year;
Console.WriteLine("Year: {0} Week: {1}", year, weekNum);
Prints out: Year: 2012 Week: 54
Change CalendarWeekRule in the above example to FirstFullWeek or FirstFourDayWeek and you'll get back 53. Let's keep the start day on Monday since we are dealing with Germany.
So week 53 starts on monday 2012-12-31, lasts one day and then stops.
53 is the correct answer. Change the Culture to germany if want to to try it.
CultureInfo cul = CultureInfo.GetCultureInfo("de-DE");
This is the way:
public int GetWeekNumber()
{
CultureInfo ciCurr = CultureInfo.CurrentCulture;
int weekNum = ciCurr.Calendar.GetWeekOfYear(DateTime.Now, CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday);
return weekNum;
}
Most important for is the CalendarWeekRule parameter.
See here:
https://msdn.microsoft.com/query/dev14.query?appId=Dev14IDEF1&l=IT-IT&k=k(System.Globalization.CalendarWeekRule);k(TargetFrameworkMoniker-.NETFramework
Since there doesn't seem to be a .Net-culture that yields the correct ISO-8601 week number, I'd rather bypass the built-in week determination altogether, and do the calculation manually, instead of attempting to correct a partially correct result.
What I ended up with is the following extension method:
/// <summary>
/// Converts a date to a week number.
/// ISO 8601 week 1 is the week that contains the first Thursday that year.
/// </summary>
public static int ToIso8601Weeknumber(this DateTime date)
{
var thursday = date.AddDays(3 - date.DayOfWeek.DayOffset());
return (thursday.DayOfYear - 1) / 7 + 1;
}
/// <summary>
/// Converts a week number to a date.
/// Note: Week 1 of a year may start in the previous year.
/// ISO 8601 week 1 is the week that contains the first Thursday that year, so
/// if December 28 is a Monday, December 31 is a Thursday,
/// and week 1 starts January 4.
/// If December 28 is a later day in the week, week 1 starts earlier.
/// If December 28 is a Sunday, it is in the same week as Thursday January 1.
/// </summary>
public static DateTime FromIso8601Weeknumber(int weekNumber, int? year = null, DayOfWeek day = DayOfWeek.Monday)
{
var dec28 = new DateTime((year ?? DateTime.Today.Year) - 1, 12, 28);
var monday = dec28.AddDays(7 * weekNumber - dec28.DayOfWeek.DayOffset());
return monday.AddDays(day.DayOffset());
}
/// <summary>
/// Iso8601 weeks start on Monday. This returns 0 for Monday.
/// </summary>
private static int DayOffset(this DayOfWeek weekDay)
{
return ((int)weekDay + 6) % 7;
}
First of all, ((int)date.DayOfWeek + 6) % 7) determines the weekday number, 0=monday, 6=sunday.
date.AddDays(-((int)date.DayOfWeek + 6) % 7) determines the date of the monday preceiding the requested week number.
Three days later is the target thursday, which determines what year the week is in.
If you divide the (zero based) day-number within the year by seven (round down), you get the (zero based) week number in the year.
In c#, integer calculation results are round down implicitly.
In .NET 3.0 and later you can use the ISOWeek.GetWeekOfDate-Method.
Note that the year in the year + week number format might differ from the year of the DateTime because of weeks that cross the year boundary.
C# to Powershell port from code above from il_guru:
function GetWeekOfYear([datetime] $inputDate)
{
$day = [System.Globalization.CultureInfo]::InvariantCulture.Calendar.GetDayOfWeek($inputDate)
if (($day -ge [System.DayOfWeek]::Monday) -and ($day -le [System.DayOfWeek]::Wednesday))
{
$inputDate = $inputDate.AddDays(3)
}
# Return the week of our adjusted day
$weekofYear = [System.Globalization.CultureInfo]::InvariantCulture.Calendar.GetWeekOfYear($inputDate, [System.Globalization.CalendarWeekRule]::FirstFourDayWeek, [System.DayOfWeek]::Monday)
return $weekofYear
}
Here is an extension version and nullable version of il_guru's answer.
Extension:
public static int GetIso8601WeekOfYear(this DateTime time)
{
var day = CultureInfo.InvariantCulture.Calendar.GetDayOfWeek(time);
if (day >= DayOfWeek.Monday && day <= DayOfWeek.Wednesday)
{
time = time.AddDays(3);
}
return CultureInfo.InvariantCulture.Calendar.GetWeekOfYear(time, CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday);
}
Nullable:
public static int? GetIso8601WeekOfYear(this DateTime? time)
{
return time?.GetIso8601WeekOfYear();
}
Usages:
new DateTime(2019, 03, 15).GetIso8601WeekOfYear(); //returns 11
((DateTime?) new DateTime(2019, 03, 15)).GetIso8601WeekOfYear(); //returns 11
((DateTime?) null).GetIso8601WeekOfYear(); //returns null
var cultureInfo = CultureInfo.CurrentCulture;
var calendar = cultureInfo.Calendar;
var calendarWeekRule = cultureInfo.DateTimeFormat.CalendarWeekRule;
var firstDayOfWeek = cultureInfo.DateTimeFormat.FirstDayOfWeek;
var lastDayOfWeek = cultureInfo.LCID == 1033 //En-us
? DayOfWeek.Saturday
: DayOfWeek.Sunday;
var lastDayOfYear = new DateTime(date.Year, 12, 31);
var weekNumber = calendar.GetWeekOfYear(date, calendarWeekRule, firstDayOfWeek);
//Check if this is the last week in the year and it doesn`t occupy the whole week
return weekNumber == 53 && lastDayOfYear.DayOfWeek != lastDayOfWeek
? 1
: weekNumber;
It works well both for US and Russian cultures. ISO 8601 also will be correct, `cause Russian week starts at Monday.
The easiest way to determine the week number ISO 8601 style using c# and the DateTime class.
Ask this: the how-many-eth thursday of the year is the thursday of this week.
The answer equals the wanted week number.
var dayOfWeek = (int)moment.DayOfWeek;
// Make monday the first day of the week
if (--dayOfWeek < 0)
dayOfWeek = 6;
// The whole nr of weeks before this thursday plus one is the week number
var weekNumber = (moment.AddDays(3 - dayOfWeek).DayOfYear - 1) / 7 + 1;
In PowerShell 7.x.y:
You will need both of the following codelines, if you have the need for the matching WeekYear.
[System.Globalization.ISOWeek]::GetWeekOfYear((get-date))
[System.Globalization.ISOWeek]::GetYear((get-date))
If you don't have .NET 5.0, extend the DateTime class to include week number.
public static class Extension {
public static int Week(this DateTime date) {
var day = (int)CultureInfo.CurrentCulture.Calendar.GetDayOfWeek(date);
return CultureInfo.CurrentCulture.Calendar.GetWeekOfYear(date.AddDays(4 - (day == 0 ? 7 : day)), CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday);
}
}
Alternative answer if you want globalized week numbers:
I also posted the answer here
From the original answer:
Built on top of this answer: by #bunny4
But not everyone is located in the US or might have to support several cultures.
Use this solution to support a cultural defined week rule and first-Day rule.. e.g. Denmark has "FirstFourDayWeek" rule for weeks and "Monday" as first day of the week.
//for now, take the the current executing thread's Culture
var cultureInfo = Thread.CurrentThread.CurrentCulture;
//let's pick a date
DateTime dt = new DateTime(2020, 12, 21);
DayOfWeek firstDay = cultureInfo.DateTimeFormat.FirstDayOfWeek;
CalendarWeekRule weekRule = cultureInfo.DateTimeFormat.CalendarWeekRule;
Calendar cal = cultureInfo.Calendar;
int week = cal.GetWeekOfYear(dt, weekRule, firstDay);
The question is: How do you define if a week is in 2012 or in 2013?
Your supposition, I guess, is that since 6 days of the week are in 2013, this week should be marked as the first week of 2013.
Not sure if this is the right way to go.
That week started on 2012 (On monday 31th Dec), so it should be marked as the last week of 2012, therefore it should be the 53rd of 2012. The first week of 2013 should start on monday the 7th.
Now, you can handle the particular case of edge weeks (first and last week of the year) using the day of week information. It all depends on your logic.
DateTimeFormatInfo dfi = DateTimeFormatInfo.CurrentInfo;
DateTime date1 = new DateTime(2011, 1, 1);
Calendar cal = dfi.Calendar;
Console.WriteLine("{0:d}: Week {1} ({2})", date1,
cal.GetWeekOfYear(date1, dfi.CalendarWeekRule,
dfi.FirstDayOfWeek),
cal.ToString().Substring(cal.ToString().LastIndexOf(".") + 1));
Based on il_guru's answer, I created this version for my own needs that also returns the year component.
/// <summary>
/// This presumes that weeks start with Monday.
/// Week 1 is the 1st week of the year with a Thursday in it.
/// </summary>
/// <param name="time">The date to calculate the weeknumber for.</param>
/// <returns>The year and weeknumber</returns>
/// <remarks>
/// Based on Stack Overflow Answer: https://stackoverflow.com/a/11155102
/// </remarks>
public static (short year, byte week) GetIso8601WeekOfYear(DateTime time)
{
// Seriously cheat. If its Monday, Tuesday or Wednesday, then it'll
// be the same week# as whatever Thursday, Friday or Saturday are,
// and we always get those right
DayOfWeek day = CultureInfo.InvariantCulture.Calendar.GetDayOfWeek(time);
if (day >= DayOfWeek.Monday && day <= DayOfWeek.Wednesday)
{
time = time.AddDays(3);
}
// Return the week of our adjusted day
var week = (byte)CultureInfo.InvariantCulture.Calendar.GetWeekOfYear(time, CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday);
return ((short)(week >= 52 & time.Month == 1 ? time.Year - 1 : time.Year), week);
}
These two methods will help, assumming our week starts on Monday
/// <summary>
/// Returns the weekId
/// </summary>
/// <param name="DateTimeReference"></param>
/// <returns>Returns the current week id</returns>
public static DateTime GetDateFromWeek(int WeekReference)
{
//365 leap
int DaysOffset = 0;
if (WeekReference > 1)
{
DaysOffset = 7;
WeekReference = WeekReference - 1;
}
DateTime DT = new DateTime(DateTime.Now.Year, 1, 1);
int CurrentYear = DT.Year;
DateTime SelectedDateTime = DateTime.MinValue;
while (CurrentYear == DT.Year)
{
int TheWeek = WeekReportData.GetWeekId(DT);
if (TheWeek == WeekReference)
{
SelectedDateTime = DT;
break;
}
DT = DT.AddDays(1.0D);
}
if (SelectedDateTime == DateTime.MinValue)
{
throw new Exception("Please check week");
}
return SelectedDateTime.AddDays(DaysOffset);
}
/// <summary>
/// Returns the weekId
/// </summary>
/// <param name="DateTimeReference"></param>
/// <returns>Returns the current week id</returns>
public static int GetWeekId(DateTime DateTimeReference)
{
CultureInfo ciCurr = CultureInfo.InvariantCulture;
int weekNum = ciCurr.Calendar.GetWeekOfYear(DateTimeReference,
CalendarWeekRule.FirstFullWeek, DayOfWeek.Monday);
return weekNum;
}
If you need the combination of calendar week and year (e.g. "1/22" for calender week 1 in 2022) be aware that the year component is not always equal to the year of the input date. This is because the first days of a year belong to the last calendar week of the previous year if at least 4 days of the week belong to last year. Accordingly the last days of a year belong to the first calendar week of the next year if at least 4 days of the week belong to next year.
Here is an extension for that based on the extension provided in the answer by #Jogge
public static class DateTimeExtensions
{
public static int GetIso8601WeekOfYear(this DateTime time)
{
var day = CultureInfo.InvariantCulture.Calendar.GetDayOfWeek(time);
if (day >= DayOfWeek.Monday && day <= DayOfWeek.Wednesday)
{
time = time.AddDays(3);
}
return CultureInfo.InvariantCulture.Calendar.GetWeekOfYear(time, CalendarWeekRule.FirstFourDayWeek,
DayOfWeek.Monday);
}
public static int? GetIso8601WeekOfYear(this DateTime? time)
{
return time?.GetIso8601WeekOfYear();
}
public static string GetIso8601WeekAndYearString(this DateTime time)
{
var week = time.GetIso8601WeekOfYear();
var year = time.Month == 1 && week >= 52
? time.Year - 1
: time.Month == 12 && week == 1
? time.Year + 1
: time.Year;
return $"{week}/{new DateTime(year, 1, 1):yy}";
}
public static string GetIso8601WeekAndYearString(this DateTime? time)
{
return time?.GetIso8601WeekAndYearString();
}
}
A year has 52 weeks and 1 day or 2 in case of a lap year (52 x 7 = 364). 2012-12-31 would be week 53, a week that would only have 2 days because 2012 is a lap year.
public int GetWeekNumber()
{
CultureInfo ciCurr = CultureInfo.CurrentCulture;
int weekNum = ciCurr.Calendar.GetWeekOfYear(DateTime.Now,
CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday);
return weekNum;
}
I was looking for a way to fetch the same day of the current week as a year ago. For example, today is:
August 10th 2022 - Wednesday.
Assume this is the check-in date, the check-out date I expect to get is:
August 11, 2021 - Wednesday.
Because it's the same day (Wednesday) as last year. But I need to take leap years into account, so I need to see if the current year is a leap year and if it is, if it has passed the 29th of February, the same with the date last year.
How to do this using .net core ? I thought of something like:
private DateTime GetDayOneYearBefore()
{
if(DateTime.IsLeapYear(DateTime.Today.Year) && DateTime.Today.Month > 2){
return DateTime.Today.AddDays(-365);
}
else if(DateTime.IsLeapYear(DateTime.Today.Year) && DateTime.Today.Month <= 2){
return DateTime.Today.AddDays(-364);
}
}
Since you mention the "same week" I suppose you want to get the same day of the week in the same week number?
If so, you can do the following:
// In the System.DayOfWeek enum Sunday = 0, while Monday = 1
// This converts DateTime.DayOfWeek to a range where Monday = 0 and Sunday = 6
static int DayOfWeek(DateTime dt)
{
const int weekStart = (int)System.DayOfWeek.Monday;
const int daysInAWeek = 7;
return (daysInAWeek - (weekStart - (int)dt.DayOfWeek)) % daysInAWeek;
}
var calendar = CultureInfo.CurrentCulture.Calendar;
var weekNum = calendar.GetWeekOfYear(DateTime.Today, CalendarWeekRule.FirstFourDayWeek, System.DayOfWeek.Monday);
var todayLastYear = DateTime.Today.AddYears(-1);
var lastYearWeekNum = calendar.GetWeekOfYear(todayLastYear, CalendarWeekRule.FirstFourDayWeek, System.DayOfWeek.Monday);
var sameWeekLastYear = todayLastYear.AddDays(7 * (weekNum - lastYearWeekNum));
var sameDaySameWeekLastYear = sameWeekLastYear.AddDays(DayOfWeek(DateTime.Today) - DayOfWeek(sameWeekLastYear));
As you might notice there's a little convertion method, since I normally work with Monday being the first day of the week. If you prefer a different day to be the first day of the week, simply replace System.DayOfWeek.Monday with which ever day you'd like.
See this fiddle for a test run.
I try to calculate the "Mid-Autumn Festival" public holiday in china for any year. But the date is for some years invalid, i use the documentation of wikipedia (15th day of 8th Lunisolar month)
var chineseCalendar = new ChineseLunisolarCalendar();
var midAutumnFestival = chineseCalendar.ToDateTime(year, 8, 15, 0, 0, 0, 0);
The date in the year 2017 and 2020 are invalid, what is wrong?
Calculated Date | Correct Date
2020.09.02 | 2020.10.01
2017.09.05 | 2017.10.04
2017 and 2020 have leap months.
https://www.chinesegenderchart.info/leapmonth.php
Leap 6th month for 2017
Leap 4th month for 2020
Now look into below documentation (https://msdn.microsoft.com/en-us/library/system.globalization.chineselunisolarcalendar%28v=vs.110%29.aspx?f=255&MSPPError=-2147217396)
A leap month can occur after any month in a year. For example, the
GetMonth method returns a number between 1 and 13 that indicates the
month associated with a specified date. If there is a leap month
between the eighth and ninth months of the year, the GetMonth method
returns 8 for the eighth month, 9 for the leap eighth month, and 10
for the ninth month.
Based on above documentation, when you query for 8th month in 2017 and 2020, you have to ask for 9 as month which represents 8th month.
something like below:
var chineseCalendar = new ChineseLunisolarCalendar();
var midAutumnFestival = chineseCalendar.ToDateTime(year, 9, 15, 0, 0, 0, 0);
I previously asked this question to take a oollection of datetime objects and group them by dayOfweek and time
So just to recap: I take a collection of DateTime
List<DateTime> collectionOfDateTime = GetDateColletion();
and then grouping by dayofWeek and time of day by doing this
var byDayOfWeek = collectionOfDateTime.GroupBy(dt => dt.DayOfWeek + "-" + dt.Hour + "-" + dt.Minute);
So at this point, I have these grouped by week (consistent time) working perfectly.
I now have a new requirement to group by Month instead of by week. When i say "month", its not the same day of the month but something like "the first tuesday of each Month"
I am trying to figure out what "key" to use in a group by to group all items that fit that monthly logic (the first tuesday of the month, the second friday of each month, etc)
As an example lets say i had these dates to start out with;
var date1 = new DateTime(2013, 1, 4).AddHours(8); // This is the first Friday in Jan
var date2 = new DateTime(2013, 2, 1).AddHours(8); // This is the first Friday in Feb
var date3 = new DateTime(2013, 1, 5).AddHours(3); // This is the first Sat in Jan
var date4 = new DateTime(2013, 2, 2).AddHours(3); // This is the first Sat in Feb
var date5 = new DateTime(2013, 2, 2).AddHours(6); // This is the first Sat in Feb - different time
If these were the dates that went into the original array, i need a groupby to end up with 3 groups.
The first group would have date1 & date2 in it
The second group would have date3 and date4 in it.
date5 would be on its own as it doesn't match any of the other groups given the different time
Can anyone suggest anyway to group by that criteria?
I think it's easier than it looks:
var byDayOfMonth = from d in dates
let h = (d.Day / 7) + 1
group d by new { d.DayOfWeek, h } into g
select g;
Local variable h = (d.Day / 7) + 1 sets which DayOfWeek within that month it actually is.
I run it for test and received 2 groups, exactly the same as in your example. Keys for that groups are:
{ DayOfWeek = Friday, h = 1 }
{ DayOfWeek = Saturday, h = 1 }
What means, there are groups for 'First Friday of month' and 'First Saturday of month'.
You can easily extend grouping key by d.Hour and/or d.Minute if you like:
var byDayOfMonth = from d in dates
let h = (d.Day / 7) + 1
group d by new { d.DayOfWeek, h, d.Hour, d.Minute } into g
select g;
Results (keys only):
{ DayOfWeek = Friday, h = 1, Hour = 8, Minute = 0 }
{ DayOfWeek = Saturday, h = 1, Hour = 3, Minute = 0 }
{ DayOfWeek = Saturday, h = 1, Hour = 6, Minute = 0 }
There is probably an easier way to do this but this is what's come to me:
I gather from your question that you need to group everything from "the first Tuesday of February until the first Monday of March" etc. such that you get these "month" spans that are a variable number of days - depending on the month in which they start. If so then you really need to break this down into ranges using the day of the year so:
Group by the First Wednesday of the Month 2013
Group 0 (0-1)
All DayOfYear between 0 and 1 2013
Group 1 (2-36)
The first Wednesday of the month: January is DayOfYear 2.
The first Wednesday of the month: February is DayOfYear 37.
etc.
So the first range is a function f such that f(32) = 1 (DayOfYear is 32) because it falls in the range 2 to 37. This f is an indexed collection of ranges, finding the item in the collection that a given DayOfYear falls into, and returning that item's index as the group number.
You can dynamically build this table by getting your min and max dates from GetDateCollection to determine the overall range. Because the logic surrounding dates is a pretty complex topic in of itself I'd fall back on a library like NodaTime (specifically the arithmetic documentation), start with the min date, advance day by day until I found the first qualifying day (i.e., "first Monday of the month") and create a range 0 to that day - 1 as group 0 and push that onto an indexed collection (ArrayList likely). Then loop from that date using LocalDate.PlusWeeks(1) until the month changes, constructing a new range and pushing that range onto the same indexed collection.
Each time you cross into a new year you'll have to add 365 (or 366 if the previous year is a leap year) to your DayOfYear as you build your indexed collection since DayOfYear resets each year.
Now you've got a collection of ranges that acts as a table that groups days into the desired units based on their DayOfYear.
Write a function that traverses the table comparing the DayOfYear (+ [365|366] * x where x is the # of years the date you are comparing is from your min year) of a given date against the items in the collection until you locate the range that day falls within, and return that index of that item as the group number. (Alternatively each range could be a Func<DateTime,bool> that returns true if the provided DateTime falls in that range.)
An alternative data structure to the collection of ranges would be an array of ushort with length equal to all the days from min to max dates in your date range, and the value for each day their assigned group number (calculated with ranges, as above). This will perform faster for grouping, though the performance may not be noticeable if you're working with a smaller dataset (only a few hundred dates).
To group by using Linq maybe this code will help you:
List<DateTime> collectionOfDateTime = GetDateColletion();
collectionOfDateTime.GroupBy(s => Convert.ToInt16(s.DayOfWeek) & s.Hour & s.Minute);
I am trying to list the next 10 weeks
The result should be like this:
Week Year
----------------
45: 2012
46: 2012
47: 2012
48: 2012
49: 2012
50: 2012
51: 2012
52: 2012
1: 2013
2: 2013
some years there is a week 53, and this year the 31st of December is on a Monday, is this week 1 or week 53??
Anyways, I want to skip week 53, whenever it occours. This means that 1 or 2 days will not be a part of any week on the list, but this doesn't matter.
some years there is a week 53, and this year the 31st of December is on a Monday, is this week 1 or week 53??
Assuming you mean "week of week-year", that would be week 1 of week-year 2013.
In the ISO calendar, the first week of a week-year is the first Monday-Sunday week which has 4 days or more in it.
It's not clear why you'd want to skip week 53 - it doesn't just skip 1 or 2 days, it skips a whole week.
Of course, this really is assuming you mean the ISO definition of "week of year". If you don't, it's a different matter. You need to clarify your requirements before you do anything else.
To obtain the week-of-week-year from .NET, you can use Calendar.GetWeekOfYear - for the ISO definition you'd use CalendarWeekRule.FirstFourDayWeek and DayOfWeek.Monday. I don't know whether there's anything to get the week-year itself though.
As a blatant plug, in my Noda Time there's support for both WeekYear and WeekOfWeekYear on dates, and you can construct a date for a given week-year/week-of-week-year/day-of-week combination.
i just wrote a small console app that does just that:
static void Main(string[] args)
{
System.Globalization.CultureInfo ci = System.Threading.Thread.CurrentThread.CurrentCulture;
DateTime dt = DateTime.Now; //Use current date as starting point
for (int i = 0; i < 10; i++)
{
int weekNo = ci.Calendar.GetWeekOfYear(
dt,
ci.DateTimeFormat.CalendarWeekRule,
ci.DateTimeFormat.FirstDayOfWeek
);
int year = ci.Calendar.GetYear(dt);
if (weekNo == 53) //if week number==53, then go to next week
{
dt = dt.AddDays(7);
weekNo = ci.Calendar.GetWeekOfYear(
dt,
ci.DateTimeFormat.CalendarWeekRule,
ci.DateTimeFormat.FirstDayOfWeek
);
year = ci.Calendar.GetYear(dt);
}
dt = dt.AddDays(7);
Console.WriteLine(weekNo + "-" + year);
}
}
output today:
46-2012
47-2012
48-2012
49-2012
50-2012
51-2012
52-2012
1-2013
2-2013
3-2013
from msdn http://msdn.microsoft.com/en-us/library/system.globalization.calendar.getweekofyear.aspx
using System;
using System.Globalization;
public class Example
{
public static void Main()
{
DateTimeFormatInfo dfi = DateTimeFormatInfo.CurrentInfo;
DateTime date1 = new DateTime(2011, 1, 1);
Calendar cal = dfi.Calendar;
Console.WriteLine("{0:d}: Week {1} ({2})", date1,
cal.GetWeekOfYear(date1, dfi.CalendarWeekRule,
dfi.FirstDayOfWeek),
cal.ToString().Substring(cal.ToString().LastIndexOf(".") + 1));
}
}
// The example displays the following output: // 1/1/2011: Week 1 (GregorianCalendar)
The following will work based off of the number of weeks since the start of the year.
if you want this to be based off of full weeks then you'll need to determine an offset of 0 or 1 at the start based on the day of the first week of the year.
public class Week
{
public Week(int weekOfYear, int year)
{
WeekOfYear = weekOfYear;
Year = year;
}
public int WeekOfYear { get; private set; }
public int Year { get; private set; }
}
public IEnumerable<Week> Next10Weeks(DateTime startDate)
{
DateTime tempDate = startDate;
for (int i = 0; i < 10; i++)
{
//add one to first parameter if you want the 1-indexed week instead of 0 indexed
yield return new Week(tempDate.DayOfYear % 7, tempDate.Year);
tempDate.AddDays(7);
}
}