I'm not sure but isn't it more effort? Adding 5 hours results in times from 24:00 to 28:00 so I think these hours must be merged anyway into next day and then into next month and at the end of a year you have 5 hours left for the next YYYY.bar. Therefore at the beginning of a year you must borrow 5 hours from the year before. Maybe there is a library for adding hours to a datetime, this would make it surely a bit easier.