[utils] Fix parsing `YYYYMMDD` dates in Nov/Dec (#2094)

The date format `%Y%m%d%H%M` will successfully match against
one-digit month, day, hour, and minute strings, even though %m et al.
are documented as being zero-padded. So dates without time in
Nov/Dec may be wrongly parsed as dates in January with time.

This commit adds a format string of `%Y%m%d` to our supported date
format strings directly below (higher priority) its problematic relatives.

Closes #2076
Authored by: wlritchi