Find if ISOWeek() is shared between two months [duplicate] - go

Question

With this simple function, I can get the week number. Now, with the number of the week, how can I get the date range, started on Sunday?
import (
"fmt"
"time"
)
func main() {
Week(time.Now().UTC())
}
func Week(now time.Time) string {
_, thisWeek := now.ISOWeek()
return "S" + strconv.Itoa(thisWeek)
}

Answer 1

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().
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

Answer 2

The following does the work of finding the first day of week for me, although not from week number but from time. If you add an extra parameter - for the hard-coded time.Monday - that can be any day of week, e.g. Sunday.
func weekStartDate(date time.Time) time.Time {
offset := (int(time.Monday) - int(date.Weekday()) - 7) % 7
result := date.Add(time.Duration(offset*24) * time.Hour)
return result
}
Test:
func TestWeekStartDate(t *testing.T) {
date := time.Date(2020, 1, 1, 0, 0, 0, 0, time.UTC)
for i := 0; i < 2000; i++ {
weekStart := weekStartDate(date)
log.Printf("%s %s", date.Format("2006-01-02 Mon"), weekStart.Format("2006-01-02 Mon"))
assert.NotNil(t, weekStart)
assert.Equal(t, time.Monday, weekStart.Weekday())
date = date.Add(24 * time.Hour)
}
}
Output:
...
2021/01/17 08:50:03 2020-12-15 Tue 2020-12-14 Mon
2021/01/17 08:50:03 2020-12-16 Wed 2020-12-14 Mon
2021/01/17 08:50:03 2020-12-17 Thu 2020-12-14 Mon
2021/01/17 08:50:03 2020-12-18 Fri 2020-12-14 Mon
2021/01/17 08:50:03 2020-12-19 Sat 2020-12-14 Mon
2021/01/17 08:50:03 2020-12-20 Sun 2020-12-14 Mon
2021/01/17 08:50:03 2020-12-21 Mon 2020-12-21 Mon
2021/01/17 08:50:03 2020-12-22 Tue 2020-12-21 Mon
2021/01/17 08:50:03 2020-12-23 Wed 2020-12-21 Mon
2021/01/17 08:50:03 2020-12-24 Thu 2020-12-21 Mon
2021/01/17 08:50:03 2020-12-25 Fri 2020-12-21 Mon
2021/01/17 08:50:03 2020-12-26 Sat 2020-12-21 Mon
2021/01/17 08:50:03 2020-12-27 Sun 2020-12-21 Mon
2021/01/17 08:50:03 2020-12-28 Mon 2020-12-28 Mon
2021/01/17 08:50:03 2020-12-29 Tue 2020-12-28 Mon
2021/01/17 08:50:03 2020-12-30 Wed 2020-12-28 Mon
2021/01/17 08:50:03 2020-12-31 Thu 2020-12-28 Mon
2021/01/17 08:50:03 2021-01-01 Fri 2020-12-28 Mon
2021/01/17 08:50:03 2021-01-02 Sat 2020-12-28 Mon
2021/01/17 08:50:03 2021-01-03 Sun 2020-12-28 Mon
2021/01/17 08:50:03 2021-01-04 Mon 2021-01-04 Mon
2021/01/17 08:50:03 2021-01-05 Tue 2021-01-04 Mon
2021/01/17 08:50:03 2021-01-06 Wed 2021-01-04 Mon
2021/01/17 08:50:03 2021-01-07 Thu 2021-01-04 Mon
2021/01/17 08:50:03 2021-01-08 Fri 2021-01-04 Mon
2021/01/17 08:50:03 2021-01-09 Sat 2021-01-04 Mon
2021/01/17 08:50:03 2021-01-10 Sun 2021-01-04 Mon
2021/01/17 08:50:03 2021-01-11 Mon 2021-01-11 Mon
2021/01/17 08:50:03 2021-01-12 Tue 2021-01-11 Mon
2021/01/17 08:50:03 2021-01-13 Wed 2021-01-11 Mon
2021/01/17 08:50:03 2021-01-14 Thu 2021-01-11 Mon
2021/01/17 08:50:03 2021-01-15 Fri 2021-01-11 Mon
2021/01/17 08:50:03 2021-01-16 Sat 2021-01-11 Mon
2021/01/17 08:50:03 2021-01-17 Sun 2021-01-11 Mon
2021/01/17 08:50:03 2021-01-18 Mon 2021-01-18 Mon
2021/01/17 08:50:03 2021-01-19 Tue 2021-01-18 Mon
2021/01/17 08:50:03 2021-01-20 Wed 2021-01-18 Mon
2021/01/17 08:50:03 2021-01-21 Thu 2021-01-18 Mon
2021/01/17 08:50:03 2021-01-22 Fri 2021-01-18 Mon
2021/01/17 08:50:03 2021-01-23 Sat 2021-01-18 Mon
2021/01/17 08:50:03 2021-01-24 Sun 2021-01-18 Mon
2021/01/17 08:50:03 2021-01-25 Mon 2021-01-25 Mon
2021/01/17 08:50:03 2021-01-26 Tue 2021-01-25 Mon
2021/01/17 08:50:03 2021-01-27 Wed 2021-01-25 Mon
2021/01/17 08:50:03 2021-01-28 Thu 2021-01-25 Mon
2021/01/17 08:50:03 2021-01-29 Fri 2021-01-25 Mon
2021/01/17 08:50:03 2021-01-30 Sat 2021-01-25 Mon
2021/01/17 08:50:03 2021-01-31 Sun 2021-01-25 Mon
2021/01/17 08:50:03 2021-02-01 Mon 2021-02-01 Mon
2021/01/17 08:50:03 2021-02-02 Tue 2021-02-01 Mon
2021/01/17 08:50:03 2021-02-03 Wed 2021-02-01 Mon
2021/01/17 08:50:03 2021-02-04 Thu 2021-02-01 Mon
2021/01/17 08:50:03 2021-02-05 Fri 2021-02-01 Mon
...

Answer 3

Thanks to #prajwal Singh, I've found more generic to find out the start and last day of the week w.r.t month, week, and year
func DateRange(week, month, year int) (startDate, endDate time.Time) {
timeBenchmark := time.Date(year, time.Month(month), 1, 0, 0, 0, 0, time.UTC)
weekStartBenchmark := timeBenchmark.AddDate(0, 0, -(int(timeBenchmark.Weekday())+6)%7)
startDate = weekStartBenchmark.AddDate(0, 0, (week-1)*7)
endDate = startDate.AddDate(0, 0, 6)
return startDate, endDate
}

Answer 4

Thanks to #icza for the solution, found a way to simplify it even further in terms of logic:
func DateRange(week, year int) (startDate, endDate time.Time) {
timeBenchmark := time.Date(year, 7, 1, 0, 0, 0, 0, time.UTC)
weekStartBenchmark := timeBenchmark.AddDate(0, 0, -(int(timeBenchmark.Weekday())+6)%7)
_, weekBenchmark := weekStartBenchmark.ISOWeek()
startDate = weekStartBenchmark.AddDate(0, 0, (week-weekBenchmark)*7)
endDate = startDate.AddDate(0, 0, 6)
return startDate, endDate
}
Works fine as well.