Purchase Order Approval Leadtimes

Hi All, 

I am in the process of doing a Generic inquiry tracking approvals and time differences between approval steps.

But I'm struggling to get the difference between one approval step and another approval step.

Step 1 Calculated Field: =iif([POApproval.StepID]='DBB0026D-FC97-ED11-9A17-00155D000300' ,[POApproval.ApproveDate] , Null)

Step 2 Calculated Field: =iif([POApproval.StepID]='423376CC-B2A9-EC11-81C1-3CECEF0440D8' ,[POApproval.ApproveDate] , Null)

Both the above returns dates, I am trying to get a datediff between the 2( Step 1 and Step 2), but my formula returns blank. 😑

Datediff Formula: =Datediff( 'dd', iif([POApproval.StepID]='DBB0026D-FC97-ED11-9A17-00155D000300',[POApproval.ApproveDate] , Null), iif([POApproval.StepID]='423376CC-B2A9-EC11-81C1-3CECEF0440D8' ,[POApproval.ApproveDate] , Null))

Anyone have any idea how I could approach getting the date difference? 

Thanks in Advance.