Where can i find java Script?

Hi everyone,

I am not a Dev and I need a code that helps me set a field based on another. 

They are both date fields

Field A and Field B

Field is an input field for a date

Field B: If field A  is 1st day of the month, add 12 months  and show first day of month after that

or If Field A is not 1st of the month, then add 13 months and show 1st day of the month 

for example:

If field A = Sep 1st, 2015

Field B = Sep 1st, 2016

but if field A = Sep 13, 2015 ( or any day other than first of the month)

then Field B = Oct 1st, 2016.

Can someone please tell me where i could get the code for this? 

thank you

E