Well, going by your logic, items can have a duration of 24.8 days.
Do you really wish to have items with THAT long of a duration?
If that's the case then wouldn't be better to just add a separate column in the player_items table, where you input the UNIX-TIME when the item was added to the player, and then have an onlogin script or something that checks if the item has "expired" and thus removes it if that's the case.