Date range by week number Golang

Foreword: Time.ISOWeek() returns you the week number that starts on Monday, so I will answer your question that also handles weeks starting on Monday. Alter it to your needs if you want it to work with weeks starting on Sunday.

I released this utility in github.com/icza/gox, see timex.WeekStart().

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The standard library does not provide a function that would return you the date range of a given week (year+week number). So we have to construct one ourselves.

And it's not that hard. We can start off from the middle of the year, align to the first day of the week (Monday), get the week of this time value, and corrigate: add as many days as the week difference multiplied by 7.

This is how it could look like:

func WeekStart(year, week int) time.Time {
    // Start from the middle of the year:
    t := time.Date(year, 7, 1, 0, 0, 0, 0, time.UTC)

    // Roll back to Monday:
    if wd := t.Weekday(); wd == time.Sunday {
        t = t.AddDate(0, 0, -6)
    } else {
        t = t.AddDate(0, 0, -int(wd)+1)
    }

    // Difference in weeks:
    _, w := t.ISOWeek()
    t = t.AddDate(0, 0, (week-w)*7)

    return t
}

Testing it:

fmt.Println(WeekStart(2018, 1))
fmt.Println(WeekStart(2018, 2))
fmt.Println(WeekStart(2019, 1))
fmt.Println(WeekStart(2019, 2))

Output (try it on the Go Playground):

2018-01-01 00:00:00 +0000 UTC
2018-01-08 00:00:00 +0000 UTC
2018-12-31 00:00:00 +0000 UTC
2019-01-07 00:00:00 +0000 UTC

One nice property of this WeekStart() implementation is that it handles out-of-range weeks nicely. That is, if you pass 0 for the week, it will be interpreted as the last week of the previous year. If you pass -1 for the week, it will designate the second to last week of the previous year. Similarly, if you pass max week of the year plus 1, it will be interpreted as the first week of the next year etc.

The above WeekStart() function only returns the given week's first day (Monday), because the last day of the week is always its first day + 6 days.

If we also need the last day:

func WeekRange(year, week int) (start, end time.Time) {
    start = WeekStart(year, week)
    end = start.AddDate(0, 0, 6)
    return
}

Testing it:

fmt.Println(WeekRange(2018, 1))
fmt.Println(WeekRange(2018, 2))
fmt.Println(WeekRange(2019, 1))
fmt.Println(WeekRange(2019, 2))

Output (try it on the Go Playground):

2018-01-01 00:00:00 +0000 UTC 2018-01-07 00:00:00 +0000 UTC
2018-01-08 00:00:00 +0000 UTC 2018-01-14 00:00:00 +0000 UTC
2018-12-31 00:00:00 +0000 UTC 2019-01-06 00:00:00 +0000 UTC
2019-01-07 00:00:00 +0000 UTC 2019-01-13 00:00:00 +0000 UTC