When building ikiwiki from a tarball, the mtime (conceptually, the
last modification date of the file) is preserved by tar, but the inode
change time (creation/metadata-change date of *this copy* of the file)
is not. This seems to lead to unstable sort ordering and
unreproducible builds.
The page can't possibly have been modified before it was created, so
we can assume that the modification date is an upper bound for the
creation date.