Get the number of weeks for a month

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How can I get the number of weeks for a month? Thereby I want to get all weeks, even the one which are not full weeks for this month.

Posted 16-Mar-15 3:07am

bfb

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Solution 3

A 30 day month has 4 2/7 weeks' worth of days, but those days may not start or end on a whole week. This you already know it being the nature of your problem.

If the first day of the month lines up with the beginning of a week - you know the month covers 5 weeks - four whole weeks and the 'spare days' starting the fifth week. For a 30 day month, provided it starts anywhere between the first and the sixth day of the week this will be true.

If the first day of the month lines up with the SEVENTH day of the week, however, you're into the 'six week' mode.

For a 31 day month, that last case includes the sixth and seventh day of the week.

For a 28 or 29 day month - I'm sure you can figure out the pattern yourself.

So - I would create four lookup tables (one each for 28,29,30,31 days in a month), each with seven entries. The entry contains the number of days in a month; you index into the table based on the weekday of the first on the month.

Posted 16-Mar-15 4:55am

Chris Ross 2

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bfb 16-Mar-15 11:40am

Thanks. Do you have a suggestion of how to implement the lookup table?

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Herman<T>.Instance · Accepted Answer · 2015-03-16T03:08:00

Solution 1

MSDN Style:

C#

DateTimeFormatInfo dfi = DateTimeFormatInfo.CurrentInfo;
Calendar cal = dfi.Calendar;

var currentWeekno = cal.GetWeekOfYear(date, dfi.CalendarWeekRule, dfi.FirstDayOfWeek);

So create a first day of month date and a last day of month date and you have the weeknumbers.

Posted 16-Mar-15 3:08am

Herman<T>.Instance

Updated 16-Mar-15 3:09am

v2

Comments

bfb 16-Mar-15 9:16am

There is a week number problematic, when the first day of a year isn't a Monday. So the question is when is the beginning of a week counted?

Also for the current month (2015-03-16) it outputs 4. It should be 6!

bfb 16-Mar-15 9:54am

I know that there are 4.4 weeks, but I need the span of the number of weeks of a month. Therefore I have to calculate all incomplete weeks at the beginning and end. In the worst case this are 6 weeks.

Richard Deeming 16-Mar-15 10:47am

digimanus's code will give you what you're looking for:

int startWeek = cal.GetWeekOfYear(new DateTime(2015, 3, 1), dfi.CalendarWeekRule, dfi.FirstDayOfWeek);
int endWeek = cal.GetWeekOfYear(new DateTime(2015, 3, 31), dfi.CalendarWeekRule, dfi.FirstDayOfWeek);
int numberOfWeeks = 1 + endWeek - startWeek; // == 6

bfb 16-Mar-15 10:54am

I tried it only in a dotnetfiddle: https://dotnetfiddle.net/pWJFgO

Here the output is 5. I don't know why. And why are you adding one week?

Richard Deeming 16-Mar-15 11:10am

Looks like that's down to a difference in the first day of week settings. My computer has Monday, whereas .NET Fiddle has Sunday. If you replace dfi.FirstDayOfWeek with DayOfWeek.Monday, you'll get six weeks.

You might also need to replace dfi.CalendarWeekRule with CalendarWeekRule.FirstDay to avoid months starting in week 53 of the previous year.

And I'm adding 1 because you want to include the start week.

bfb 16-Mar-15 11:53am

So for localization I should stick with `dfi` and if I don't want a localization I take the values you proposed?

Richard Deeming 16-Mar-15 11:55am

Yes - using dfi will take the rules for the current user's culture.