Whoops.

It would appear that this isn't needed.  MTEntryModifiedDate got added in 3.something; I was just unaware of it as mod_gzip has been selectively not updating static archives of pages, including every single one in my docs directory.  Therefore, I was still using the documentation from 2.63 as opposed to 3.17

I probably just sent everybody's (ok, probably just Peter's) feed readers into a tizzy.

All those entries showing up as modified?  Yeah, they aren't really modified.  It's just that MT doesn't, by default, allow access to the mt_entry.entry_modified_on field, and I was initially too lazy to type up the half-dozen lines of Perl that would allow me to use that instead of mt_entry.entry_created_on.  It shouldn't happen again.

For anyone who looks at the Perl code and goes "That's not a half-dozen lines", well:  most of it is copied from another plugin I wrote.  Only about a half-dozen lines had to be changed to make it do what I wanted (not counting deleted lines).