diff --git a/Makefile b/Makefile
index 69902783..ceeaa2e6 100644
--- a/Makefile
+++ b/Makefile
@@ -104,7 +104,7 @@ force-upgrade:
 	rm -f .venv/upgraded-on
 	$(MAKE) upgrade
 upgrade-quiet:
-	echo "Automatically generated by Python Package Makefile on `date --rfc-3339=seconds`." > .venv/upgraded-on
+	echo "Automatically generated by Python Package Makefile on `date '+%Y-%m-%d %H:%M:%S %z'`." > .venv/upgraded-on
 
 # Generate a Software Bill of Materials (SBOM).
 .PHONY: sbom