Populate an option set when date field is populated.

I wonder if someone could help.

I have a datefield on a contact form which is stated as Birthday (schema: "birthdate") in MSD 2015

When the date field is populated with the contacts birthday, i have an option set with all the months of the year in the pick list.

So i want the month in the date field to populate the option set with the correct month.

i.e. - Birthday: 01/01/1987 (Date field)

- Birthday Month: January (option set)

Can anyone help me with a Jscript as mine does not seem to be working.

Thanks