Function MONTH returning 11 for the period dec

Good morning,

The function MONTH is returning in our model the number 11 for the month of December, the number 12 for the month of January and number 1 for February. Anyone know why is this happening? and why december is not the 12 month and januray the 1 month?

Find screenshoots attached:

We have a calendar type 4-4-5 as below:

Answers