Simon B Simon B - 2 months ago 7
C# Question

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

, which applies the rules I need, but by overriding the first day of year in a
that derives from say,

Unfortunately I can't find a way of doing this. Adding an arbitrary 122 / 123 days to the date in question will break the
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?


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