Ok, you made me look up an example: "The Ethiopian calendar has twelve months of thirty days plus five or six epagomenal days, which comprise a thirteenth month

Ok, you made me look up an example: "The Ethiopian calendar has twelve months of thirty days plus five or six epagomenal days, which comprise a thirteenth month." https://en.wikipedia.org/wiki/Intercalation_(timekeeping) https://en.wikipedia.org/wiki/Intercalation_(timekeeping) https://en.wikipedia.org/wiki/Ethiopian_calendar https://en.wikipedia.org/wiki/Ethiopian_calendar I guess its the choice if you fix the offset every year or wait until you accumulate a full month of offset.