Question

I can only find algorithm for getting ISO 8601 week (week starts on a Monday).

However, the iCal spec says

A week is defined as a seven day period, starting on the day of the
week defined to be the week start (see WKST). Week number one of the
calendar year is the first week that contains at least four (4) days
in that calendar year.

Therefore, it is more complex than ISO 8601 since the start of week can be any day of the week.

Is there an algorithm to determine what is the week number of a date, with a custom start day of week?

or… is there a function in iCal4j that does this? Determine a weekno from a date?

Thanks!

p.s. Limitation: I’m using a JVM language that cannot extend a Java class, but I can invoke Java methods or instantiate Java classes.

Answer 1

1. Let daysInFirstWeek be the number of days on the first week of the year that are in January. Week starts on a WKST day. (e.g. if Jan 1st is a WKST day, return 7)

2. Set dayOfYear to the n-th days of the input date’s year (e.g. Feb 1st = 32)

3. If dayOfYear is less than or equal to daysInFirstWeek

   3.1. if daysInFirstWeek is greater than or equal to 4, weekNo is 1, skip to step 5.

   3.2. Let daysInFirstWeekOfLastYear be the number of days on the first week of the previous year that are in January. Week starts on a WKST day.

   3.3. if daysInFirstWeekOfLastYear is 4 or last year is Leap year and daysInFirstWeekOfLastYear is 5, weekNo is 53, otherwise weekNo is 52, skip to step 5.

4. Set weekNo to ceiling((dayOfYear - daysInFirstWeek) / 7)

   4.1. if daysInFirstWeek greater than or equal to 4, increment weekNo by 1

   4.2. if daysInFirstWeek equal 53 and count of days on the first week (starting from WKST) of January in the year of inputDate‘s year + 1 is greater than or equal to 4, set weekNo to 1

5. return weekNo