For one user date calculation results differs from another users

[Karmy]

Hi,

in my application there is a due date field calculated by baseline date and payment term which are stored in database. Baseline date's format in db is "dd.mm.yyyy". 

Calculation logic is simple:

At first the application changes the dates to format "mm/dd/yyyy" and then adds hard -coded amount of date units according to the payment term code. Finally the calculated date is reformatted back to the original format.

Now, the problem is that for one user the calculation results differs from another users and thus are incorrect.   

Does anyone have a clue what is happening here?