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.
This question talks about validating a string representing a date, and in it folks mention that it's good to avoid using Exceptions for regular flow logic. And TryParse() is great for that. But TryParse() takes a string, and in in my case i've already got the year month and day as integers. I want to validate the month/day/year combination. For example February 30th.
It's pretty easy to just put a try/catch around new DateTime(int, int, int), but I'm wondering if there's a way to do it without relying on exceptions.
I'd also feel silly composing these ints into a string and then using TryParse().
The following will check for valid year/month/day combinations in the range supported by DateTime, using a proleptic Gregorian calendar:
public bool IsValidDate(int year, int month, int day)
{
return year >= 1 && year <= 9999
&& month >= 1 && month <= 12
&& day >= 1 && day <= DateTime.DaysInMonth(year, month);
}
If you need to work with other calendar systems, then expand it as follows:
public bool IsValidDate(int year, int month, int day, Calendar cal)
{
return year >= cal.GetYear(cal.MinSupportedDateTime)
&& year <= cal.GetYear(cal.MaxSupportedDateTime)
&& month >= 1 && month <= cal.GetMonthsInYear(year)
&& day >= 1 && day <= cal.GetDaysInMonth(year, month);
}
Use String Interpolation
int year = 2017;
int month = 2;
int day = 28;
DateTime dt;
DateTime.TryParse($"{month}/{day}/{year}", out dt);
As far as I know, there's no easy way to check for a DateTime's int's validity besides concatenating the ints into a correctly formatted string beforehand.
To avoid try/catch-ing, I would write a static utility class which utilizes DateTime.TryParse:
using System;
public static class DateTimeUtilities
{
public static bool TryParse(int year, int month, int day, out DateTime result)
{
return DateTime.TryParse(
string.Format("{0}/{1}/{2}", year, month, day), out result);
}
}
Usage:
DateTime dateTime;
if (DateTimeUtilities.TryParse(2017, 2, 30, out dateTime))
{
// success
}
else
{
// fail, dateTime = DateTime.MinValue
}
Pending the needs of your application, e.g. culture (thanks #Matt Johnson), I would also look into DateTime.TryParseExact.
Look at it this way. Any code that you write:
Will have to check month ranges 1-12
Will have to check day ranges by month, which means you'll have to hard code an array
Will have to account for leap years, which can be a pain the the rear
Rather than doing ALL that, and reinventing the wheel, and potentially getting it wrong -- why don't you keep it simple and just wrap the DateTime constructor in a try-catch and keep it moving? Let the nerds up in Redmond do all the hard work for this common task. The best solution is one that any developer following you can understand and rely upon quickly.
I'd bet money that under the hood, TryParse and the DateTime constructor are using the exact same validators, except that the latter throws an exception while the former does not. TryParse, for this, is overkill with all the extra string manipulation involved.
I want to compare a given date to today and here is the condition: If provided date is greater than or equal to 6 months earlier from today, return true else return false
Code:
string strDate = tbDate.Text; //2015-03-29
if (DateTime.Now.AddMonths(-6) == DateTime.Parse(strDate)) //if given date is equal to exactly 6 months past from today (change == to > if date has to be less 6 months)
{
lblResult.Text = "true"; //this doesn't work with the entered date above.
}
else //otherwise give me the date which will be 6 months from a given date.
{
DateTime dt2 = Convert.ToDateTime(strDate);
lblResult.Text = "6 Months from given date is: " + dt2.AddMonths(6); //this works fine
}
If 6 months or greater than 6 months is what I would like for one
condition
If less than 6 months is another condition.
Your first problem is that you're using DateTime.Now instead of DateTime.Today - so subtracting 6 months will give you another DateTime with a particular time of day, which is very unlikely to be exactly the date/time you've parsed. For the rest of this post, I'm assuming that the value you parse is really a date, so you end up with a DateTime with a time-of-day of midnight. (Of course, in my very biased view, it would be better to use a library which supports "date" as a first class concept...)
The next problem is that you are assuming that subtracting 6 months from today and comparing it with a fixed date is equivalent to adding 6 months to the fixed date and comparing it with today. They're not the same operation - calendar arithmetic just doesn't work like that. You should work out which way you want it to work, and be consistent. For example:
DateTime start = DateTime.Parse(tbDate.Text);
DateTime end = start.AddMonths(6);
DateTime today = DateTime.Today;
if (end >= today)
{
// Today is 6 months or more from the start date
}
else
{
// ...
}
Or alternatively - and not equivalently:
DateTime target = DateTime.Parse(tbDate.Text);
DateTime today = DateTime.Today;
DateTime sixMonthsAgo = today.AddMonths(-6);
if (sixMonthsAgo >= target)
{
// Six months ago today was the target date or later
}
else
{
// ...
}
Note that you should only evaluate DateTime.Today (or DateTime.Now etc) once per set of calculations - otherwise you could find it changes between evaluations.
Try with this
DateTime s = Convert.ToDateTime(tbDate.Text);
s = s.Date;
if (DateTime.Today.AddMonths(-6) == s) //if given date is equal to exactly 6 months past from today (change == to > if date has to be less 6 months)
{
lblResult.Text = "true"; //this doesn't work with the entered date above.
}
replace == with >= or <= according to your needs
This is a question of best practices. I have a utility that takes in a two digit year as a string and I need to convert it to a four digit year as a string. right now I do
//DOB's format is "MMM (D)D YY" that first digit of the day is not there for numbers 1-9
string tmpYear = rowIn.DOB.Substring(rowIn.DOB.Length - 3, 2); //-3 because it is 0 indexed
if (Convert.ToInt16(tmpYear) > 50)
tmpYear = String.Format("19{0}", tmpYear);
else
tmpYear = String.Format("20{0}", tmpYear);
I am sure I am doing it horribly wrong, any pointers?
The .NET framework has a method that does exactly what you want:
int fourDigitYear = CultureInfo.CurrentCulture.Calendar.ToFourDigitYear(twoDigitYear)
That way you will correctly adhere to current regional settings as defined in Control Panel (or group policy):
Given that there are people alive now born before 1950, but none born after 2010, your use of 50 as the flipping point seems broken.
For date of birth, can you not set the flip point to the 'year of now' (i.e. 10) in your app? Even then you'll have problems with those born before 1911...
There's no perfect way to do this - you're creating information out of thin air.
I've assumed DOB = date-of-birth. For other data (say, maturity of a financial instrument) the choice might be different, but just as imperfect.
You can also use the DateTime.TryParse method to convert your date. It uses the current culture settings to define the pivot year (in my case it is 2029)
DateTime resultDate;
Console.WriteLine("CultureInfo.CurrentCulture.Calendar.TwoDigitYearMax : {0}", System.Globalization.CultureInfo.CurrentCulture.Calendar.TwoDigitYearMax);
DateTime.TryParse("01/01/28", out resultDate);
Console.WriteLine("Generated date with year=28 - {0}",resultDate);
DateTime.TryParse("01/02/29",out resultDate);
Console.WriteLine("Generated date with year=29 - {0}", resultDate);
DateTime.TryParse("01/03/30", out resultDate);
Console.WriteLine("Generated date with year=30 - {0}", resultDate);
The output is:
CultureInfo.CurrentCulture.Calendar.TwoDigitYearMax : 2029
Generated date with year=28 - 01/01/2028 00:00:00
Generated date with year=29 - 01/02/2029 00:00:00
Generated date with year=30 - 01/03/1930 00:00:00
If you want to change the behavior you can create a culture with the year you want to use as pivot. This thread shows an example
DateTime.TryParse century control C#
But as martin stated, if you want to manage a time period that spans more than 100 year, there is no way to do it with only 2 digits.
I think Java has a good implementation of this:
http://java.sun.com/javase/6/docs/api/java/text/SimpleDateFormat.html#year
People rarely specify years far into the future using a two-digit code. The Java implementation handles this by assuming a range of 80 years behind and 20 years ahead of the current year. So right now, 30 would be 2030, while 31 would be 1931. Additionally, this implementation is flexible, modifying its ranges as time goes on, so that you don't have to change the code every decade or so.
I just tested, and Excel also uses these same rules for 2-digit year conversion. 1/1/29 turns into 1/1/2029. 1/1/30 turns into 1/1/1930.
The implementation of
System.Globalization.CultureInfo.CurrentCulture.Calendar.ToFourDigitYear
is
public virtual int ToFourDigitYear(int year)
{
if (year < 0)
throw new ArgumentOutOfRangeException("year", Environment.GetResourceString("ArgumentOutOfRange_NeedNonNegNum"));
if (year < 100)
return (this.TwoDigitYearMax / 100 - (year > this.TwoDigitYearMax % 100 ? 1 : 0)) * 100 + year;
else
return year;
}
Hope this helps!
It might be smarter to check tmpYear > currentYear%100. If it is, then it's 19XX, otherwise 20XX.
This solution we use for Expiration Dates, the user enters MM and YY into separate fields. This results in dates being the 31st or 30th and 28th or 29th also for February.
/// <summary>
/// Creates datetime for current century and sets days to end of month
/// </summary>
/// <param name="MM"></param>
/// <param name="YY"></param>
/// <returns></returns>
public static DateTime GetEndOfMonth(string MM, string YY)
{
// YY -> YYYY #RipVanWinkle
// Gets Current century and adds YY to it.
// Minus 5 to allow dates that may be expired to be entered.
// eg. today is 2017, 12 = 2012 and 11 = 2111
int currentYear = DateTime.Now.Year;
string thisYear = currentYear.ToString().Substring(0, 2) + YY;
int month = Int32.Parse(MM);
int year = Int32.Parse(thisYear);
if ((currentYear - 5) > year)
year += 100;
return new DateTime(year, month, DateTime.DaysInMonth(year, month));
}
This Method can convert the credit card last two year digits to four year
private static int ToFourDigitYear(int year)
{
string stringYear = year.ToString("00");
if (stringYear.Length == 2)
{
int currentYear = DateTime.Now.Year;
string firstTwoDigitsOfCurrentYear = currentYear.ToString().Substring(0, 2);
year = Convert.ToInt32(firstTwoDigitsOfCurrentYear + stringYear);
if (year < currentYear)
year = year + 100;
}
return year;
}
Out of curiosity, from where do you get this data? From a form? In that case; I would simply ask the user to fill in (or somehow select) the year with four digits or get the users age and month/day of birth, and use that data to figure out what year they were born. That way, you wouldn't have to worry about this problem at all :)
Edit: Use DateTime for working with this kind of data.
Try this simple code
//Invoke TextBoxDateFormat method with date as parameter.
Method
public void TextBoxDateFormat(string str1)
{
// Takes the current date format if MM/DD/YY or MM/DD/YYYY
DateTime dt = Convert.ToDateTime(str1);
//Converts the requested date into MM/DD/YYYY and assign it to textbox field
TextBox = String.Format("{0:MM/dd/yyyy}", dt.ToShortDateString());
//include your validation code if required
}
Had a similar issue, and came up with this... HTH!
value = this.GetDate()
if (value.Length >= 6)//ensure that the date is mmddyy
{
int year = 0;
if (int.TryParse(value.Substring(4, 2), out year))
{
int pastMillenium = int.Parse(DateTime.Now.ToString("yyyy").Substring(0, 2)) - 1;
if (year > int.Parse(DateTime.Now.ToString("yy")))//if its a future year it's most likely 19XX
{
value = string.Format("{0}{1}{2}", value.Substring(0, 4), pastMillenium, year.ToString().PadLeft(2, '0'));
}
else
{
value = string.Format("{0}{1}{2}", value.Substring(0, 4), pastMillenium + 1, year.ToString().PadLeft(2, '0'));
}
}
else
{
value = string.Empty;
}
}
else
{
value = string.Empty;
}
My answer will not match your question but for credit cards I just add 2 digits of current year
private int UpconvertTwoDigitYearToFour(int yearTwoOrFour)
{
try
{
if (yearTwoOrFour.ToString().Length <= 2)
{
DateTime yearOnly = DateTime.ParseExact(yearTwoOrFour.ToString("D2"), "yy", null);
return yearOnly.Year;
}
}
catch
{
}
return yearTwoOrFour;
}
If you calculate for a person he will probably not be more than 100 years...
Eg: 751212
var nr = "751212";
var century = DateTime.Now.AddYears(-100).Year.ToString().Substring(0, 2);
var days = (DateTime.Now - DateTime.Parse(century + nr)).Days;
decimal years = days / 365.25m;
if(years>=99)
century = DateTime.Now.Year.ToString().Substring(0, 2);
var fullnr = century+nr;
To change a 2-digit year to 4-digit current or earlier -
year = year + (DateTime.Today.Year - DateTime.Today.Year%100);
if (year > DateTime.Today.Year)
year = year - 100;
My two cents,
Given an age range=[18, 100+], two digits year=90, I can do
current year - twoDigitsYear = 2018 - 90 = 1928, I got 19, 28
hence 19 is the first two digits of year of born, and 28 is the age, which is
year=1990, age=28
But it won't work when age 0 and 100 both included in the range, same to some of the other answers here.
Based on above solutions, here is mine, i used in android while using java
it takes current year in two digit format then checks for if input
year length is equal to 2, if yes then it get current year and from
this year it splits first two digits of century, then it adds this
century with year user input. to make it 4 digit year.
public static int getConvertedYearFromTwoToFourDigits(String year) {
if (year.length() == 2) {
int currentYear = Calendar.getInstance().get(Calendar.YEAR);
String firstTwoDigitsOfCurrentYear = String.valueOf(currentYear).substring(0, 2);
year = firstTwoDigitsOfCurrentYear + year;
}
return Integer.parseInt(year);
}
int fYear = Convert.ToInt32(txtYear.Value.ToString().Substring(2, 2));
My answer will not match your question but for credit cards I just add 2 digits of current year
private int UpconvertTwoDigitYearToFour(int yearTwoOrFour)
{
try
{
if (yearTwoOrFour.ToString().Length <= 2)
{
DateTime yearOnly = DateTime.ParseExact(yearTwoOrFour.ToString("D2"), "yy", null);
return yearOnly.Year;
}
}
catch
{
}
return yearTwoOrFour;
}