More robust date handling.

Previously the date handling expiry logic had a bug where if you had
backups that were a year apart to the day, it wouldn't notice the
difference in year and only notice that the month was the same, and
expire the older one (eg, if you had a backup on 2012-04-01 and
another on 2013-04-01, it'd delete the one from 2012. This commit
makes it compare the full date string instead of just the month, so
that it more robustly keeps older backups.