Calculate Difference between two date times.

I need to be able to calculate the difference between two date fields on my grid.

I have built an employee timesheet app.

They have a start and finish time I want to work out how many hours between the two times correct to 2 decimal places. can someone please help me?

These are the column indexes.
var colStartTime = 6
var colFinishTime = 8

Any Help would be greatly appreciated.