If providing a customized active topics since an individual user last "logged in" is proving to be more difficult than expected, perhaps there's an easier to provide function that would serve many people's needs. Just giving me the active topics since yesterday 12:01am would suffice. That way, I know I can catch all the active threads if I just check FT every day at any time during the day. Only providing active threads since 12:01am today means that I need to check as late at night as possible, plus then if everybody does the same thing, many of the posts will then occur late at night, and possibly missed by somebody who wasn't willing to stay up until 11:59pm.

A similar spin would be to provide the active topics in the last N hours, rather than using day boundaries at all.