How does mknmz determine if a file is updated?

How does mknmz determine if a file is updated?  Looking at the source,
it appears it is based solely on mtime.  Is this true?

A problem I am try to solve is if mknmz can be told to re-index
a file that has changed, but the mtime is the same.  Is there
a way to do this?  Is the -Z option the solution?

To give some context, I have a mail archive that is organized into
period-based mhonarc archives.  I would like to be able to re-index
a given period (all periods are a single search index) without having
to rebuild the entire search index from scratch.

--ewh