Scuby, go vote for this Idea and then I'll give you the (probable) answer:
PS: Just kidding, if you're formatting is exactly as you described you could use RIGHT([my-date-time],12]). But it's probably not, or at least T probably isn't bringing it in that way. So you'll need to post a spread sheet and/or packaged workbook (it can contain only your Date/Time field, don't really need anything else). Also best if you have some rows in both a.m. and p.m.