Adding this to at least partially prevent the virtually nonexistent possibility of timestamp and _utc_date() mismatch. If the normal date -u -d does not get converted (looking at you Solaris!), the poor man's method with manipulating the _utc_date() string output kicks in.
Corrects issue #3285.
The '?' character after a group is not supported in POSIX Basic Regular Expressions. Replacing it with '\{0,1\}' retains the same functionality and also works on non-GNU systems.