CODEWITHSUNDEEP
c#
Simon B
Simon B

Reputation: 1096

Calculating the week number for a date, when your year starts on a non 1st January day

I need to calculate the week number for a given date, when my year begins on the 1st of May.

My hope was to use Calendar.GetWeekOfYear(), which applies the rules I need, but by overriding the first day of year in a Calendar that derives from say, GregorianCalendar.

Unfortunately I can't find a way of doing this. Adding an arbitrary 122 / 123 days to the date in question will break the GetWeekOfYear() CalendarWeekRule.FirstFourDayWeek rule I wish to use.

Short of writing a custom and I suspect messy algorithm to determine this, is there a way of calculating the week number for a given date, when my year begins on the 1st of May?

Upvotes: 0

Views: 107

Answers (2)

Simon B
Simon B

Reputation: 1096

I ended up with this, which works but doesn't handle the week 53 scenario:

    private int calculateBizWeekNumber(DateTime date, DateTime yearStart, CalendarWeekRule cwr, DayOfWeek firstDoW)
    {
        var cal = DateTimeFormatInfo.CurrentInfo.Calendar;

        var dateWoY = cal.GetWeekOfYear(date, cwr, firstDoW);
        var firstWoY = cal.GetWeekOfYear(yearStart, cwr, firstDoW);

        var bizWeek = dateWoY - firstWoY + 1;

        return bizWeek <= 0 ? 52 + bizWeek : bizWeek;
    }

Upvotes: 0

Tim Schmelter
Tim Schmelter

Reputation: 460058

Maybe you could use this approach which simply subtracts the 121 days from January 1st to May 1st from a given DateTime and then uses Calendar.GetWeekOfYear on that date:

static readonly int DayOffset = (new DateTime(DateTime.Today.Year, 5, 1) 
                               - new DateTime(DateTime.Today.Year, 1, 1)).Days; // 121

// ... somewhere else:
DateTime date = DateTime.Today;
DateTime offsetDate = date.AddDays(-DayOffset);
int weekNum = CultureInfo.CurrentCulture.Calendar.GetWeekOfYear(offsetDate, CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday); 
Console.WriteLine("Year:{0} Week:{1}", offsetDate.Year, weekNum); // Year:2016 Week:19

Upvotes: 1

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