Check if a datetime is in same week as other datetime

Question

Let's say I have a list of dates in a table. Now I want to find all rows, which is in the same week as the date provided as an argument.

Let's say I have a table with:

• 2014-09-11
• 2014-09-12
• 2014-09-15

And I give this function the argument 2014-09-09, it has to look from monday->sunday, and realize that 09-11 and 09-12 is part of the week, but not 09-15.

How on earth to solve this?

I have thought on making a check on year, month and weeknumber, but you cannot guarantee that month is the same...

So how do you solve a problem like this?

Solution 1

DxCk's comment is valid. This solution will work even if the first or last week of the year cross two different years:

Check that the first day of the week for both dates fall on the same day. Here is the code:

    private bool DatesAreInTheSameWeek(DateTime date1, DateTime date2)
    {
        var cal = System.Globalization.DateTimeFormatInfo.CurrentInfo.Calendar;
        var d1 = date1.Date.AddDays(-1 * (int)cal.GetDayOfWeek(date1));
        var d2 = date2.Date.AddDays(-1 * (int)cal.GetDayOfWeek(date2));

        return d1 == d2;
    }

Solution 2

why not?

bool AreFallingInSameWeek(DateTime date1, DateTime date2)
{
    return date1.AddDays(-(int)date1.DayOfWeek) == date2.AddDays(-(int)date2.DayOfWeek);
}

if you want to use any day other than Sunday as start of the week

bool AreFallingInSameWeek(DateTime date1, DateTime date2, DayOfWeek weekStartsOn)
{
    return date1.AddDays(-GetOffsetedDayofWeek(date1.DayOfWeek, (int)weekStartsOn)) == date2.AddDays(-GetOffsetedDayofWeek(date2.DayOfWeek, (int)weekStartsOn));
}

int GetOffsetedDayofWeek(DayOfWeek dayOfWeek, int offsetBy)
{
    return (((int)dayOfWeek - offsetBy + 7) % 7)
}

Solution 3

Check the DateTime.Year and Calendar.GetWeekOfYear(DateTime, ...). No need to check for the month.

EDIT: This is wrong but I can't delete it. See @Sparrow's answer below.

Solution 4

Use: public virtual int GetWeekOfYear(DateTime time,CalendarWeekRule rule,DayOfWeek firstDayOfWeek) of Calendar class

Solution 5

My requirement was to find DOBs falling on the current week. Hope this helps with your doubt. Basically the idea behind this code is as follows:

1. Change the DOB to current year birthday (eg; 24-02-1988 to 24-02-2018(current year).
2. Add a year, if the brithday week contains both dec and jan
3. Get the first day of today's week.
4. Get last day of today's week.

5. check if the current year birthday falls between first day and last day of today's week.

   private bool DatesAreInTheSameWeek(DateTime birthday)
   {
       if (birthday == DateTime.MinValue)
       {
           return false;
       }
       else
       {
           var birtdayWithCurrentYear = new DateTime(DateTime.Today.Year, birthday.Month, birthday.Day);
           if (birthday.Month == 1 && DateTime.Today.Month != 1)
           {
               birtdayWithCurrentYear = birtdayWithCurrentYear.AddYears(1);
           }
           DateTime firstDayInWeek = DateTime.Today.Date;
           while (firstDayInWeek.DayOfWeek != CultureInfo.CurrentCulture.DateTimeFormat.FirstDayOfWeek)
               firstDayInWeek = firstDayInWeek.AddDays(-1);var lastDayInWeek = firstDayInWeek.AddDays(7);
   
           return birtdayWithCurrentYear < lastDayInWeek && birtdayWithCurrentYear >= firstDayInWeek;
   
       }
   }
   

View more solutions

Comments

[DxCK]

How about 31/12/2015 and 01/01/2016 that occur in the same week but different years?

[Electric Sheep]

This will only work if you want the week to start from Sunday.

[Simon Hänisch]

@OllieP you can fix that easily by subtracting 1 from the day of week: date1.AddDays(-((int)date1.DayOfWeek-1)). This should be the accepted answer imo.

[Shoejep]

You should be wary of using these in .Net core as the milliseconds on the DateTime will cause almost always a return value of false, i.e. AreFallingInSameWeek(DateTime.Now, DateTime.Now, DayOfWeek.Monday) will return false, but you can solve this by adding .Date, like this: date1.AddDays(-GetOffsetedDayofWeek(date1.DayOfWeek, (int)weekStartsOn)).Date == date2.AddDays(-GetOffsetedDayofWeek(date2.DayOfWeek, (int)weekStartsOn)).Date;

[Arve Systad]

I'd rather just create a tiny helper method that returns appropriate numbers for weekdays where I'm from than "magically subtracting" from DayOfWeek, as this requires people to know that the built in Enum starts on Sunday.

[SHS]

@Sparrow, your solution does not do the job properly in the countries, where weeks start from Monday. Try the above code with 1/11/2020 and 6/11/2020

[david-so]

For the first of the two solutions presented, you get false because it is also comparing the time. Just add .Date at the end of both values to compare just the day as follow: return date1.AddDays(-(int)date1.DayOfWeek).Date == date2.AddDays(-(int)date2.DayOfWeek).Date;

[Peter Thaus]

I tried this and had some trouble with the date as the date to check contained a time also. So I added startDateOfWeek.AddHours(-startDateOfWeek.Hour); to remove the hours, same with minutes and then the endDateOfWeek = startDateOfWeek.AddDays(7d); so I have the date starting on midnight and ending on midnight one week later. This worked fine for me. Thanks!

[Yiorgos]

You only want to compare dates up to day precision (not hours etc), so the final line should be: return d1.Date == d2.Date;