Beginning of Month error

[lvndry]

Reproducible Example

https://play.golang.org/p/o21N9NbVLdk

Description

If you run this on the 31st of the month it won't print the beginning of the next month but the beginning of the month after
So on 03-21-2021 it'll print 05-01-201 instead of 04-01-2021

[tc-hib]

Hello,

In your example, AddDate(0, 1, 0) returns 05-01-2021.
This is the expected behavior. From Time.AddDate:

AddDate normalizes its result in the same way that Date does, so, for example, adding one month to October 31 yields December 1, the normalized form for November 31.

You can fix it by adding a month after finding the first day of the month.
playground