How to calculate difference between two dates and get it converted to seconds

You can try to the following steps below on your rule:

Steps:

Add path aliases for the two cells that you would want to compare
Convert both of their time values into epoch time by using the parseDate() function
Set the difference to the cell

Refer to below screenshot:

Converting the values this way will spare you from calculating the HH:MM:SS manually.

You may read more about Rules in Chapter 11 and Time Formatting Parsing Codes here.