It’s completely done, expect one part: The timer configuration page.
Internally there are two types of timers in OH core. Absolute date/time timers and cron expression timers (recurring events etc). The first part was easy: A date/time picker.
I underestimated how complex it is to build a simple to use UI to create all variations of cron expressions.
I’m using this page as reference on what is possible with cron expressions.