Thanks Rupor! Glad I asked before messing with my database.
I did figure the number was a timestamp for the last time the book was opened. That's why I thought it was tied to the "new" badge -- i.e., if the field was null, that meant the book had never been opened and would therefore get a "new" badge. At least, that was my thinking.
But I hadn't thought of all the other things the timestamp was used for, which might get messed up if I started adding my own timestamps -- especially if I wasn't sure how those timestamps were formatted. But if one could figure out the format then create a reasonable timestamp ... ???
Then again, inputting all those timestamps may take longer than simply manually opening and closing all my epubs.