Convert a date to ISO8601 date format

Question

This is more of a PHP question than a WordPress one.

I have a custom post type event that by default outputs the date as "September 11, 2011 9:00 am" format.

I'm trying to format the post in http://schema.org/Event format, I have converted every other part of the post to that format (place,address, etc) except the dates.

Any help on how to convert that format to ISO8601 (http://en.wikipedia.org/wiki/ISO_8601) date format ?

Thanks in advance.