From <https://github.com/bmwiedemann/psm/commit/98acae1a6f2c9b2e6014eac4070a817c9d0fd8a2>.

From 98acae1a6f2c9b2e6014eac4070a817c9d0fd8a2 Mon Sep 17 00:00:00 2001
From: "Bernhard M. Wiedemann" <bwiedemann@suse.de>
Date: Sun, 23 Jul 2017 07:02:28 +0200
Subject: [PATCH] Allow to override build date

in order to make builds reproducible.
See https://reproducible-builds.org/ for why this is good
and https://reproducible-builds.org/specs/source-date-epoch/
for the definition of this variable.

Also uses UTC to be independent of timezone settings.
---
 Makefile       | 2 +-
 ipath/Makefile | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/Makefile b/Makefile
index d79c4bd..99406ef 100644
--- a/Makefile
+++ b/Makefile
@@ -270,7 +270,7 @@ ${TARGLIB}.so.${MAJOR}: ${TARGLIB}.so.${MAJOR}.${MINOR}
 # file around.  Generate it such that the ident command can find it
 # and strings -a | grep InfiniPath does a reasonable job as well.
 ${TARGLIB}.so.${MAJOR}.${MINOR}: ${${TARGLIB}-objs}
-	date +'char psmi_infinipath_revision[] ="$$""Date: %F %R ${rpm_extra_description}InfiniPath $$";' > ${lib_build_dir}/_revision.c
+	date -u -d@$${SOURCE_DATE_EPOCH:-$$(date +%s)} +'char psmi_infinipath_revision[] ="$$""Date: %F %R ${rpm_extra_description}InfiniPath $$";' > ${lib_build_dir}/_revision.c
 	$(CC) -c $(BASECFLAGS) $(INCLUDES) _revision.c -o _revision.o
 	$(CC) $(LDFLAGS) -o $@ -Wl,-soname=${TARGLIB}.so.${MAJOR} -shared -Wl,--unique='*fastpath*' \
 		${${TARGLIB}-objs} _revision.o -L$(build_dir)/ipath $(LDLIBS)
diff --git a/ipath/Makefile b/ipath/Makefile
index 8c2cc6e..73abd6f 100644
--- a/ipath/Makefile
+++ b/ipath/Makefile
@@ -70,7 +70,7 @@ ${TARGLIB}.so.${MAJOR}: ${TARGLIB}.so.${MAJOR}.${MINOR}
 # file around.  Generate it such that the ident command can find it
 # and strings -a | grep InfiniPath does a reasonable job as well.
 ${TARGLIB}.so.${MAJOR}.${MINOR}: ${${TARGLIB}-objs}
-	date +'static __attribute__ ((unused)) char __psc_infinipath_revision[] ="$$""Date: %F %R ${rpm_extra_description}InfiniPath $$";' > _revision.c
+	date -u -d@$${SOURCE_DATE_EPOCH:-$$(date +%s)} +'static __attribute__ ((unused)) char __psc_infinipath_revision[] ="$$""Date: %F %R ${rpm_extra_description}InfiniPath $$";' > _revision.c
 	$(CC) -c $(BASECFLAGS) $(INCLUDES) _revision.c -o _revision.o
 	$(CC) -o $@ -Wl,-soname=${TARGLIB}.so.${MAJOR} -shared \
 		-Wl,--unique='*fastpath*' \
amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2021-11-16 14:34:28 +0100'>2021-11-16</span></td><td><a href='/guix/commit/config-daemon.ac?id=472a0e82a52a3d5d841e1dfad6b13e26082a5750'>daemon: Do not deduplicate files smaller than 8 KiB.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>