> There doesn't appear to be any decent way to compare the last modified
> times of files from the shell...

Before everybody starts inventing their own names for this, it should be
noted that V8 already has a program for this, newer(1).  It takes two
filenames as arguments, and exits with status 0 if and only if either
(a) the first exists and the second does not, or (b) both exist and the
first's modification time is at least as recent as the second's.  Other-
wise it exits with non-zero status.  (The preceding two sentences are
essentially the whole of the manual page for it.)
 
Relatively few people have V8, but in the absence of any other precedent
for what this facility should like look, it seems reasonable to follow
V8's lead:

newer file1 file2

exit with 0 status if file1 exists and file2 does not, or if file1's last
modified time is at least as recent as file2's.