I was trying to visualize what you are intending. Are you taking '2013-11-21 17:50:42 +0000', converting it to epoch (1385056242), then attempting to find if it is a square number, and deleting if not? And you are doing the backups on 15m intervals, so you are looking for sqrt(15*$m + $d) == int(sqrt(15*$m + $d))? For the time given above, the previous and two following times that would meet the criteria are:

Thu Nov 21 10:44:16 2013 (37216)
Fri Nov 22 07:24:49 2013 (32717)
Sat Nov 23 04:05:24 2013 (32718)
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The only ways I could see that working are either:

1. you start at the oldest, deleting until you reach your threshold or a lower limit of versions to keep, or
2. you look to see if backup falls into some kind of interval that agrees with your idea (for instance, it is the closest backup to the actual time that matches).

Personally, if you are not already using it I would probably look at using --link-dest=DIR option, where you give each backup run the directory of the previous as the parameter for this option. If the files match, then they are hard-linked, so (on *nix systems, at least) they only add a directory entry in the new directory.

Hope that helps.