Unexpected date to number calculation

Hi there,

I am trying to do some calculations based on month and date. Unfortunately the system is calculating every 1st day of the month as the last day of the previous month.

Example:
01/01/2021 would be calculated in numbers as 31 12 2020. Another example would be 01/02/2021 as 31 01 2021. 

Calculating 02/01/2021 would result into 01 01 2021.

Statement:
it would always calculate 1 day in the past.

When I use default functions to do a calculation and the results end-up wrong. Support said it's not a bug but have not yet explain to me why the following is happening:
- MONTH(01/01/2021) would result into the number 12.
- DAY(01/01/2021) would result into the number 31.
- YEAR(01/01/2021) would result into the number 2020. -> only when its the 1st of january.

How does this happen and how can this be prevented because making quarterly reports is not doable now.

thanks in advance