That's because [Date Modified] is a system time, that is a large number representing the date last modified whereas [Last Played] (with which you can do the <1441 trick) shows the number of hours/days since last played, rather than showing the date.
So a [Date Modified]=<1440 is actually looking for files last modified on a set date (1440 I'm guessing would be around 1970?) rather than a set time ago.
Have a field that stored the time difference since last modified would be handy, but I don't think there's a way to do it at the moment.
Even more handy would be something similar to Wiki's... a log that stored the last modifications and dates, so you could change back if someone changed data in the wrong way (this would take more database space though... depending on how far back you kept the changes, etc... actually, I'd probably keep it in it's own database... anyway, I'm rambling :) ... I might add this to the v10 feature request page... just in case :))