formula to output upcoming quarter end date (for a broken fiscal year)

I am looking to create a function that outputs the upcoming quarter end date based on a specified start date, for which the quarter end is based on a broken fiscal year ending december 15.

As an example, say that you sign up as a customer with an internet provider on 2014-01-01. The internet provider charges all their clients on a quarterly basis and have a broken fiscal year ending on december 15. Hence, as you signed up on 2014-01-01 you will be charged on 2014-03-15, which is the date of the company's first quarter end.

So what I would like to do is to set up a function that outputs the first date I will be charged based on the date that I sign up. If I sign up between december 2013-12-16 and 2014-03-15, the formula should output 2014-03-15. If I sign up between 2014-03-16 and 2014-06-15, the formula should output 2014-06-15 etc. etc.

I would really appreciate any helpful comments.

Many thanks!