The comment currently implies that a zero will be added, but the
underscore is used to add a space for single-digit dates.

Change-Id: Ib3bac8a16bc2d1fcb26ab3bb7ad172b89e1a4a24
Reviewed-on: https://go-review.googlesource.com/75230Reviewed-by: Ian Lance Taylor <iant@golang.org>