But the difference between 2021-01-01 and 2020-12-31 is only one day, not a year. It gets even strange for e.g. select DATEDIFF(year, '2021-12-01', '2022-01-01

But the difference between 2021-01-01 and 2020-12-31 is only one day, not a year. It gets even strange for e.g. select DATEDIFF(year, '2021-12-01', '2022-01-01') which still returns 1 even though that's a whole month. I don't see a use case for this kind of result