Custom date function in drupal 6

Hi
I have a question from you.

I create an Custom function in the name Jdate() . in get standard date format and change it in to my date format in my local.

for example it get 2008 and change it to 1385 ( Persian date standard ) .

now I want to use it for change base or shown date function .

what is your suggestion for it ?

edit date function in common.php file or edit themplate.php file .

with editing common I have a changing basically in my drupal system and I don't like it but I think this way is better than template way.

I need to your suggestion.

what is the your suggestion about it .