Set last-modified time of a (local) file?

[Kai Grossjohann]

Is there a way in Emacs to set the last-modified time of a (local)
file?

I need this for Tramp, more specifically for its handling of
copy-file.  If using an inline method, then Tramp works by reading the
contents of the remote file into a buffer, and then using write-region
to write NEWNAME.  But if the KEEP-DATE arg of copy-file is set to t,
then that file written with write-region needs to get another
last-modification timestamp.

(For the case where NEWNAME is remote, I've already written a function
which invokes {/bin,/usr/bin}/touch.)

Kai