PROBLEM EXPERIENCED:

PHPBB treats empty user_dateformat fields as a custom date/time format, which causes the date displayed on forum indexes to not show.

This is particularly the case when converting to PHPBB3, the user_dateformat column can sometimes be left blank for users (eg: SMF->PHPBB).

EXPECTED BEHAVIOUR:

That in the absence of a date format value in a user's user_dateformat property, a sane default to be used (the global forum date format, perhaps?).

MORE INFORMATION:

This bug was discovered when converting from SMF->PHPBB2->PHPBB3, discussion and analysis can be found here:

http://www.phpbb.com/community/viewtopic.php?f=46&t=548727