I think he means that the length of a month varies throughout the year.

January has 31
February has 28/29
March has 31
April has 30
etc

Plus, although years are technically the same length 365.25, "1 year" isn't typically measured that way. Years are measured as 365/year plus 1 extra day on leap years.

It would be difficult and inefficient to set it up that way.

-genius_at_work