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ISO 8601 week number - Custom week start day? [duplicate]

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;
}

Get same day of same week in last year

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.

How to get the date of a given day in a specific week?

I want to get date of specific day of a given week. The week is decided by the entered date.
For example, if I want the Friday of these dates:
06/27/2018
07/04/2018
07/07/2018
I expect as outcome:
06/29/2018
07/06/2018
07/06/2018
Here, week is defined as Monday to Sunday.

You can do this with some clever maths based on the DayOfWeek:
public DateTime GetDay(DateTime source, DayOfWeek dayOfWeek,
DayOfWeek weekStartsOn = DayOfWeek.Monday)
{
var offset = (int)source.DayOfWeek - (int)weekStartsOn;;
if(offset < 0)
{
offset = offset + 7;
}
return source.AddDays(-offset + (int)dayOfWeek - (int)weekStartsOn);
}
And use it like this:
var someDate = ...; //Get a date from somewhere
var friday = GetDay(someDate, DayOfWeek.Friday);
var monday = GetDay(someDate, DayOfWeek.Monday);
And if your week starts on a Sunday, just use the optional third parameter, for example:
var friday = GetDay(someDate, DayOfWeek.Friday, DayOfWeek.Sunday);

A modification of the current version of DavidG's answer:
static DateTime GetDay(DateTime source, DayOfWeek dayOfWeek)
{
const int offsetSinceMondayIsFirstDayOfWeek = 7 - (int)DayOfWeek.Monday;
return source.AddDays(((int)dayOfWeek + offsetSinceMondayIsFirstDayOfWeek) % 7
- ((int)source.DayOfWeek + offsetSinceMondayIsFirstDayOfWeek) % 7);
}
This takes into account that the asker considers Monday to be the first day of the week. If you want Saturday as the first day of week, just replace Monday with Saturday above.
In the special case where you consider Sunday the first day of the week, it reduces to DavidG's original method:
static DateTime GetDay(DateTime source, DayOfWeek dayOfWeek)
{
return source.AddDays((int)dayOfWeek - (int)source.DayOfWeek);
}
because the DayOfWeek enum type of the BCL already "wraps around" between Saturday (= 6) and Sunday (= 0).
On http://chartsbin.com/view/41671 there is a world map that apparently shows what day of week is considered the first day of the week in different regions.

public void TestMethod1 ( )
{
DateTime date = DateTime.Now;
DateTime friday = date.AddDays( (int)DayOfWeek.Friday - (int)date.DayOfWeek );
Console.WriteLine( friday.ToString( ) );
}

using System;
using System.Globalization;
public class Example
{
public static void Main()
{
DateTime dateValue = new DateTime(2008, 6, 11);
Console.WriteLine(dateValue.ToString("ddd",
new CultureInfo("fr-FR")));
}
}

How to get the month depending on the week number in C# and .NET

How can I get the month according to a specified week number? For example, if a get the week number 2 return the month 1 (January)?

Take the week number and multiply it by 7. For example, if it is week number 12, multiply 12 by 7. This will indicate the number of days it has been, rounded to the nearest week.
Divide this number by 30. In the example, the number we derived from the calculation was 84. 84 divided by 30 is 2.8
Use this number to figure out the month. 2.8 means that it is March. The number before the decimal point indicates the months past. Always round the number up, this will give you the current month.
Read more: http://www.ehow.com/how_7444970_calculate-month-week-number.html#ixzz2zYJVK4S8

Well, first we should come to terms; since week can start in one month and ends in second month, let's find out in which month the given week starts and ends in separate methods.
Let's implement WeekStart: given a date we want to find out the corresponding week start date:
private static DateTime WeekStart(DateTime date, DayOfWeek firstDayOfWeek) {
DayOfWeek current = date.DayOfWeek;
return (current >= firstDayOfWeek)
? date.Date.AddDays(firstDayOfWeek - current)
: date.Date.AddDays(-7 + (int)firstDayOfWeek - (int)current);
}
Then we can try to guess the right week; then we have to correct it (take previous or next week when required):
private static DateTime NthWeekStart(
int year, int week, CalendarWeekRule rule, DayOfWeek firstDayOfWeek) {
DateTime guess = new DateTime(year, 1, 1).AddDays((week - 1) * 7);
// Or CurrentCulture
int actual = CultureInfo.InvariantCulture.Calendar.GetWeekOfYear(
guess, rule, firstDayOfWeek);
return WeekStart(guess.AddDays((week - actual) * 7), firstDayOfWeek);
}
Finally, we can get corresponding months:
private static int WeekStartMonth(
int year, int week, CalendarWeekRule rule, DayOfWeek firstDayOfWeek) {
return NthWeekStart(year, week, rule, firstDayOfWeek).Month;
}
private static int WeekEndMonth(
int year, int week, CalendarWeekRule rule, DayOfWeek firstDayOfWeek) {
return NthWeekStart(year, week, rule, firstDayOfWeek).AddDays(6).Month;
}
Demo:
int year = 2020;
int week = 36;
CalendarWeekRule rule = CalendarWeekRule.FirstDay;
DayOfWeek day = DayOfWeek.Sunday;
int monthStart = WeekStartMonth(year, week, rule, day);
int monthEnd = WeekEndMonth(year, week, rule, day);
Console.Write($"{week}th week of {year} starts in month #{monthStart} ends in month #{monthEnd}");
Outcome:
36th week of 2020 starts in month #8 ends in month #9

Get datetime from given day

user just enter the day of week. For instance user enter friday. I need to find the exact date of given day and format will be like dd.MM.yyyy.
But I don't know how I do it.
Example:
label1 - Friday (entered by user)
label2 - 08.06.2012 (found by system)
label1 is just a string (just Friday). It's coming from webservice as a string variable. I need to find the date and compare with today, If it's not equal or small than today I give date of upcoming Friday, else I give the date of the Friday the week after.

"If it's not equal or small than today I give exact date, else I give next week date. "
Assuming that means that you return always the next date in future with the given day of week, the only exception is when today is the given day of week.
public static DateTime getNextWeekDaysDate(String englWeekDate)
{
var desired = (DayOfWeek)Enum.Parse(typeof(DayOfWeek), englWeekDate);
var current = DateTime.Today.DayOfWeek;
int c = (int)current;
int d = (int)desired;
int n = (7 - c + d);
return DateTime.Today.AddDays((n >= 7) ? n % 7 : n);
}
Let's test:
DateTime Monday = getNextWeekDaysDate("Monday"); // 2012-06-11
DateTime Tuesday = getNextWeekDaysDate("Tuesday"); // 2012-06-05 <-- !!! today
DateTime Wednesday= getNextWeekDaysDate("Wednesday"); // 2012-06-06
DateTime Thursday = getNextWeekDaysDate("Thursday"); // 2012-06-07
DateTime Friday = getNextWeekDaysDate("Friday"); // 2012-06-08

Create enum of days (i.e. monday - 0, tuesday - 1, etc);
Get DateTime.Now DayOfWeek and cast in some way it to your enum value.
Calculate difference between Now.DayOfWeek and user's day of the week;
Use DateTime.AddDays(difference).DayofTheWeek;

get current time with DateTime.now
Current day is DateTime.Now.DayOfWeek
Then get the day of week your user entered
Then your result is DateTime.now.AddDays( NowDayOfWeek - UserDayOfWeek).