Incremental Loading of Data

My company has a process that runs every 30 minutes or so. This process loads data into the database.

I need to write a process that captures cumulative data from a particular column and compares it to the last load.
So for example: Load 1, employee 123 had a total of 25 seconds. Load 2 this same employee had a total of 30 seconds.

On each load, I need to calculate the difference of the total time for each employee. Load1 25sec, Load2 30sec --> The increment will be 5 sec.

I only need to calculate the current load with the last load.

Is there any easy way to do this? I'm stumped.

Thanks,
Ninel