The following describes the necessary Functions and Procedures to create a small "Sleep Timer Library" for OpenHAB (Don't know, if Library is the correct title, but you know what I mean ;-)). Using this approach, it's easy for you to create your own custom sleep timers.
NOTE: This stuff doesn't depend on any add-on.
The usage example does not cover initialization or recovery on system start and demonstrates a VERY BASIC example. But it's not very hard for you to integrate and customize it to your needs ;-)