Programmatically Change System Job Postpone Until

CRM Online 2015 Update 1.

When I update the Postpone Until value I am seeing strange behavior.  The Postpone Until value changes but when it hits the new value, it automatically re-adds the configured time.  Is this a bug, expected behavior or do I need to updated Postpone Until with a different approach? Here is an example with a 10 minute timeout.

The workflow runs initially at 8:48 am, setting the Postpone Until value to 8:58 am.

I programmatically change Postpone Until to 8:54 am

At 8:54 am, CRM automatically adds 10 minutes to the current time setting it to 9:04 am