Time.AddDate and monthly boundaries

A couple of days ago I ran into a weird issue: I needed to run some test-cases with dates from different months as input. Everything worked fine but then, on March 29, it all of a sudden didnโ€™t anymore.

The code looked like this:

now := time.Now()
previousMonth := now.AddDate(0, -1, 0)

On March 28 now.Month() was 3 and previousMonth.Month() was 2. Midnight came and the results were 3 and 3. This happened because, as documented, time.Date normalises values:

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. โ€” time.go

So what did happen exactly in my case? Go internally created this new Time object:

time.Date(2022, 2, 29, ...)

2022-02-29 doesnโ€™t exist but Go tries to normalise this value by calculating a UNIX timestamp and the result is actually 2022-03-01 which is, obviously, in the same month as March 29.

So what was the workaround in my case? My taste-dates are now always on the first day of the month and not based on time.Now() ๐Ÿ˜…