Be more strict about dates we generate

Be more strict about dates we generate

ISO-8601 dates must have a 4-character year component, however strftime does
not left-pad years to 4 characters when the century is one character long even
though the man page for strftime(3) suggests otherwise:

       %F     Equivalent to %Y-%m-%d (the ISO 8601 date format). (C99)

This makes stricter ISO-8601 parsers, such as Perl's DateTime module, unhappy.
So, we'll do it ourselves using the glibc extensions available to strftime for
specifying a padding character and desired length.

Signed-off-by: Mike Rylander <mrylander@gmail.com>