Maybe the fieldformat TIMESTAMP could help you: it contains data in the form yyyymmddhhmmss, which is very easy to parse and to calculate (you can use > and <, if you need to). But the big magic with this field is, when you assign NULL to it, it will contain the current timestamp.
I use this solution a lot (e.g. for a Javascript/CGI-Chat I'm just playing around, because I'm interested how it can work; you may test it on http://chat.fabiani.net/chat/, although it is by far not finished yet.

Best regards,
perl -le "s==*F=e=>y~\*martinF~stronat~=>s~[^\w]~~g=>chop,print"