Go - Unix timestamp for first day of the week from ISO year week

Question

What's a good way of getting the timestamp for, say, the Monday 00:00:00 for the fifth week of 2010? Essentially I'm looking for the inverse function to ISOweek in the time package. I can't seem to find a way to achieve that. Parse() doesn't read week, and simply counting how many "7 days" there have been since the first day of the year/month doesn't really get me what I need.

Thanks a lot in advance! Bill

Answer 1

I don't see why

simply counting how many "7 days" there have been since the first day of the year/month doesn't really get me what I need

Here is my attempt:

func firstDayOfISOWeek(year int, week int, timezone *time.Location) time.Time {
    date := time.Date(year, 0, 0, 0, 0, 0, 0, timezone)
    isoYear, isoWeek := date.ISOWeek()
    for date.Weekday() != time.Monday { // iterate back to Monday
        date = date.AddDate(0, 0, -1)
        isoYear, isoWeek = date.ISOWeek()
    }
    for isoYear < year { // iterate forward to the first day of the first week
        date = date.AddDate(0, 0, 1)
        isoYear, isoWeek = date.ISOWeek()
    }
    for isoWeek < week { // iterate forward to the first day of the given week
        date = date.AddDate(0, 0, 1)
        isoYear, isoWeek = date.ISOWeek()
    }
    return date
}

Here is working example with sanity check: http://play.golang.org/p/UVFNFcpaoI

The code is easy to understand, there is no year/month/day arithmetic, just simple iteration. The runtime complexity is O(n), but given that n is always less than 400 - it's actually not a problem at all.

It should not have problems with daylight saving, leap years and other tricky date problems if golang date.AddDate is implemented correctly.

P.S. Funny fact :) First day of week 1 of year 2008 is 2007-12-31. And this is correct, you can check in the calendar.