The function phorum_date() returns not expected formatted date/times when the local from the language file ($PHORUM['locale']) is not installed/generated in the operating system. In this case date/times are formatted with the default locale from the OS.
I think there should be a warning for the administrator of the forum.
At least as an additional hint in the language file but perhaps also as a 'system sanity check' (as part of the Language Support Check) or an event in the Event Logging Module.
It's easy to catch the return value from setlocale and compare it to $PHORUM['locale'].