Calc code pointer appreciated

Hi,

I am working on forecast functions and need to 'detect' date intervals (e.g. months).

When entering 1/1 /15in cell A1 and 1/2/15 in cell A2 I can expand this in the cells below (A3 then 
contains 1/3/15, ..., A13 1/1/16, etc.).
To avoid double and/or inconsistent code, I would like to know where the above mentioned 
month-detection takes place in the code.

Any clues?

Thanks!

Winfried