When patch updates a file, it normally sets the file’s
last-modified timestamp to the current time of day. If you are using
patch to track a software distribution, this can cause
make to incorrectly conclude that a patched file is out of
date. For example, if syntax.c depends on syntax.y, and
patch updates syntax.c and then syntax.y, then
syntax.c will normally appear to be out of date with respect to
syntax.y even though its contents are actually up to date.
The --set-utc (-Z) option causes patch to
set a patched file’s modification and access times to the timestamps
given in context diff headers. If the context diff headers do not
specify a time zone, they are assumed to use Coordinated Universal
Time (UTC, often known as GMT).
The --set-time (-T) option acts like -Z or --set-utc, except that it assumes that the context diff headers’ timestamps use local time instead of UTC. This option is not recommended, because patches using local time cannot easily be used by people in other time zones, and because local timestamps are ambiguous when local clocks move backwards during daylight-saving time adjustments. If the context diff headers specify a time zone, this option is equivalent to --set-utc (-Z).
patch normally refrains from setting a file’s timestamps if
the file’s original last-modified timestamp does not match the time
given in the diff header, of if the file’s contents do not exactly
match the patch. However, if the --force (-f)
option is given, the file’s timestamps are set regardless.
Due to the limitations of the current diff format,
patch cannot update the times of files whose contents have
not changed. Also, if you set file timestamps to values other than
the current time of day, you should also remove (e.g., with ‘make
clean’) all files that depend on the patched files, so that later
invocations of make do not get confused by the patched
files’ times.