I'm trying to figure out how to calculate a date/time by subtracting a given number of hours from a given due date/time, so that I know when I need to start a process step on a product component in order to have it finished in time to ship to the customer.

For example, the product needs to be ready to ship to the customer by 15:00 on September 15, 2017

The final assembly and verification steps, from last to first, are

* Step 15 - requires 12.6 hours to complete
* Step 10 - requires 32.1 hours to complete
* Step 05 - requires 25.9 hours to complete

working backwards

* step 15 would need to be complete by 15:00 on 9/15/2017 to ship to customer
* step 10 would need to be complete by 02:24 on 9/15/2017 to start step 15 on time
* step 05 would need to be complete by 06:18 on 9/13/2017 to start step 10 on time
* All components would need to be ready for assembly by 04:24 on 9/12/2017 to start step 05 on time

I've spent the day searching for an example that would be close enough to what I want and I think I will have to do something with the DateTime function, I'm just not sure what at the moment