Time between two Field

[Sxbbb]

I have a field  'WorkOrderNo' ,'Shift', 'Check_Time_Start' 

If Shift = 'Morning' I want Check_Time_Start subtracts with (06:00:00) to get a value between 06:00:00 and Check_Time_Start. 

So I created two fields.

I tried using the code : Check_Time_StartMachine - OverTimeMorning

Result :

if the time is between StartOverTimeMorning and OverTimeMornin 9 I want to the value to be 'Check_Time_StartMachine'