Time Based Workflow Configuration - Service Requests

I am trying to create time based workflows to fire email notifications to ensure  comply with SLA timelines.  The requirement is that Service Reps get an email 90 minutes, 105 minutes and 120 minutes after a new Service Request is created if it meets the following criteria: a)Status=Open AND b) Owner is NULL.

I have written a workflow and set up 3 wait actions with 3 corresponding email notifications.  Two of my wait actions are working perfectly because I am able to use the 'Period' section to define the wait time (15 minutes each).  However, the initial wait time is 90 mintues so I used the 'Period Expression' section and wrotw the following: Duration('P0Y0M0DT1H30M')  This wait action is not being followed, it is as if the workflow is skipping it entirely and going directly to the second wait action, which is 15 mintues.