See https://github.com/systemd/systemd/issues/2956 - it appears that `OnCalendar=*-*-* 6,18:00` and `OnCalendar=*-*-* 6:00,18:00` are quite different. Git-Dch: ignore
@@ -2,7 +2,7 @@
Description=Daily apt activities
[Timer]
-OnCalendar=*-*-* 6:00,18:00
+OnCalendar=*-*-* 6,18:00
RandomizedDelaySec=12h
AccuracySec=1h
Persistent=true