How to Calculate a Date Range Given a Week Number in Golang?

Date Range by Week Number in Golang

Background:

Time.ISOWeek() returns the week number that starts on Monday. This article demonstrates how to obtain the date range for a given week, assuming it starts from Sunday.

Solution:

1. Calculate the Week Start:

Start by aligning to the first day of the week (Monday) from the middle of the year. Corrigate by adding days based on the week difference multiplied by 7.

func WeekStart(year, week int) time.Time {
    t := time.Date(year, 7, 1, 0, 0, 0, 0, time.UTC)
    if wd := t.Weekday(); wd == time.Sunday {
        t = t.AddDate(0, 0, -6)
    } else {
        t = t.AddDate(0, 0, -int(wd)+1)
    }
    _, w := t.ISOWeek()
    t = t.AddDate(0, 0, (week-w)*7)
    return t
}

2. Calculate the Week Range:

To get the date range, determine the week's first day and add 6 days to obtain the last day.

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

Example:

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

Additional Note:

The WeekStart() function manages out-of-range weeks. Weeks outside the year range are interpreted as the last or first week of the previous or subsequent year, respectively.

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