Dating your painting is traditionally done with the 60 year cycle.  Each year (from Chinese New Year onward) is identified by 2 characters.  The next 10 years are:

Your can write just the 2 characters, or the 2 plus year (年 nian).

Once you have the characters, you need to find a calligraphy script version to write on your paintings.  If you don’t have a script dictionary, this is a useful site.  Put in the characters (in Chinese simplified script) and choose which script you want.