Add a couple conditional fields to web form, some javascript

Have an existing web form that needs 1 more madatory field, and a conditional date field based on the 1st field's answer. The madatory field will ask if any money has been paid yet to date and the answer will be a simple dropdown select with "yes/no" options; If "yes" is chosen then the conditional date field will appear(and will be mandatory now) below the yes/no question and ask for a date that would be the current date or a past date, using the same javascript calendar that is currently used with 2 other date fields in the form. This date field will act exactly like the other 2 date fields - either you can choose a date from the popup calendar or fill it in by hand. Javascript code has to be added to check this field for a valid date that is the current date or a past date before submitting the form. If "no" is chosen then no conditional field is shown and the user can continue on filling in the rest of the web form. Looking for a javascript/jQuery/html pro who can jump in and figure out from the existing code how best to efficiently handle adding these fields using the existing form and javascript structure.