You can test OnCalendar strings using systemd-analyze calendar, eg:
# systemd-analyze calendar '*-*-* 06:00:00,18:00:00'
Failed to parse calendar specification '*-*-* 06:00:00,18:00:00':
Invalid argument
# systemd-analyze calendar '*-*-* 06,18:00:00'
Normalized form: *-*-* 06,18:00:00
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