Short Version:

I want to do the following:

If an entry has been modified, and if that modification took place more than ten minutes after the original posting, then show "Last update at $MTEntryModifiedDate$".

Less-Short Version:
I'm currently using the Compare plugin like this:
But because I almost always make small changes in the few minutes after posting, this will almost always result in the "Last Updated" text being displayed, when I really only want it displayed if I've made a real change (i.e., if my modification occurred more than 10 minutes after the original post -- arbitrary time period, but it fits my habits.)

I was hoping to do this sans PHP, possibly using some combo of Compare and Arithmetic. (Ideally I'd just do it within Compare, but it doesn't do the math.) I think I could use Arithmetic's MTSetVar2 tag and Compare's MTIfGreaterThan tag to accomplish this. In theory, it should work; in practice, it doesn't.

I think the fundamental problem is that, unformatted, $MTEntryDate$ doesn't return a numerical string (like the PHP date() function would), and so the Arithmetic tags can't work on them. So my question to you is: how would you achieve my goal? Would you force the date tags to return a string by formatting them as such? If so, how would you format them, and when you subtracted one from the other, what value would indicate ten minutes? Is there a better way altogether to go about this?