When using the ISO-8601 standard for defining recurring timer events (ex. "R3/PT30M"), each job DueDate is calculated by adding the defined time period ("PT30M") to the start time of the first job (if it's not already defined in the expression) at most "R3" times.

If the DueDate of a timer job is ever modified, this doesn't impact the due dates of the following timer jobs.

AT:
It should be possible to calculate the next job due date from the execution time of the previous job.