What is the appropriate way to present relative vs absolute date selection to the user?
In [increment/decrement numeric field] [mins/hours dropdown]
On [calendar-selector], [time-selector]
Your absolute date selection doesn't need any other changes; that's straight forward:
When it comes to the relative date selection, I would change 2 things:
The blank field should probably be a spinner control that increments and decrements integer values.
I would add a link to the side that would allow you to add another date part. It would then add another spinner control and drop down with the remaining available date parts. This would give the users a more precise granularity.