Related note: with systemd you are probably better off using its OnCalendar facilities. There is even a tool called 'systemd-analyze', which can be used for verifying the correctness of your OnCalendar timespec:
$ systemd-analyze calendar "*:11/5" --iterations 3
Original form: *:11/5
Normalized form: *-*-* *:11/5:00
Next elapse: Mon 2021-08-30 23:21:00 EEST
(in UTC): Mon 2021-08-30 20:21:00 UTC
From now: 47s left
Iter. #2: Mon 2021-08-30 23:26:00 EEST
(in UTC): Mon 2021-08-30 20:26:00 UTC
From now: 5min left
Iter. #3: Mon 2021-08-30 23:31:00 EEST
(in UTC): Mon 2021-08-30 20:31:00 UTC
From now: 10min left
