The line numbers have changed in the newer versions of the files, so can't precisely locate the error in ProblemSetDetail.pm.
The errors in Problem.pm and ProblemSetList.pm are most likely related and occur when a comparison is made to see if the current time is before after or between the open date, due date and/or answer date.
If you check the problem sets for which these errors occur (by looking at them via Hmwk Sets Editor -> Edit problems ---- title at the top of the page will say "Set Detail for set ...." ) you will probably find that the date information at the top of the page is either missing or has some kind of bad format. (format should look like
01/10/1997 at 06:00am EST
Could you report back which version of webwork you are using (rel-2-3 , rel-2-2 ?).
I think we've caught most instances in which the dates are accidentally left undefined -- but one can never be sure.