Go: Number of days in month

Aug 27, 2021

How do you get the number of days in a given month in Go?

Go’s time package has an internal daysIn function to do this, but it’s not exported for general use. Why? “Because simplicity” according to Googlers. The preferred version of “simplicity” is to have ten thousand programs reimplement it separately instead.

This single line function will do the trick:

import "time"

func daysIn(m time.Month, year int) int {
    return time.Date(year, m+1, 0, 0, 0, 0, 0, time.UTC).Day()

The reason it works is that we generate a date one month on from the target one (m+1), but set the day of month to 0. Days are 1-indexed, so this has the effect of rolling back one day to the last day of the previous month (our target month of m). Calling Day() then procures the number we want.

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