Steps to reproduce:

1. Open Configuration -> Maintenance
2. Press Create.
3. Go to Periods tab.
4. Click "Add" and select Period type except "One time only"
5. Type 12:12 in field "At (hour:minute)"
6. Press Apply
7. Edit period

Result:
"At (hour:minute)" is set to 0:0
Expected:
"At (hour:minute)" is 12:12

For Period type "Monthly" is reset field "Date" on "Day of month"