Adobe Pro Calculations of drop down menus

This question has been asked many times. And there are several ways to perform this calculation. 

Time values are not regular calculations, because the user does not enter a time value, they enter a string value that represents a time. 

You can read the basics about time and date handling here:

https://www.pdfscripting.com/public/Date-and-Time-Handling.cfm

However, since you are using a dropdown for entering the dates, you have a unique opportunity to make life easier for yourself.  You can make the export values of the input dropdowns the actual time values. For example "8:00" is 8 hours after midnight, so make it the value 8.  "14:30" is 12.5 hours after midnight, so make the export value 12.5.

Next, make the Shift Hours field a read only text field. 

Use this custom calculation script:

event.value = this.getField("Finish").value - this.getField("Dropdown7").value;

There are more issues that have to be handled to create a complete solution, but getting the calculation right is the first bit.