Day 0 is as invalid as day 32.

Fixes #17874

Change-Id: I52109d12bafd6d957d00c44d540cb88389fff0a7
Reviewed-on: https://go-review.googlesource.com/33429
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>