Golang: Reverse ISOWeek, get the date of the first day of ISO week

If you work with weeknummers, you can find the weeknumber of a date with the Golang function time.ISOWeek. However you wish to get the first date of a week, there is no such function in Google Go.┬áSo let’s write it our selfs:

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()

// iterate back to Monday
for date.Weekday() != time.Monday {
date = date.AddDate(0, 0, -1)
isoYear, isoWeek = date.ISOWeek()
}

// iterate forward to the first day of the first week
for isoYear < year {
date = date.AddDate(0, 0, 7)
isoYear, isoWeek = date.ISOWeek()
}

// iterate forward to the first day of the given week
for isoWeek < week {
date = date.AddDate(0, 0, 7)
isoYear, isoWeek = date.ISOWeek()
}

return date
}