Reference `until` when calculating next occurrence

According to RFC 5545:

> The UNTIL rule part defines a DATE or DATE-TIME value that bounds the
> recurrence rule in an inclusive manner.  If the value specified by
> UNTIL is synchronized with the specified recurrence, this DATE or
> DATE-TIME becomes the last instance of the recurrence.

[1] http://tools.ietf.org/html/rfc5545