Converting 4 digit number to time unit and extrapolating time to run from there

Hello all

Part One
I am trying to find a way to create a macro that will convert the numbers in column B, on the Master worksheet, of the attached file from a number to its time. For example I would like the number 1224 in B1 to be 12:24.

Part Two
Once the above conversion is done I then need to get the difference between the times shown going down the column. What I mean is that B1 is the start of a test and B2 is the start of a new test, and B3 is a start of the third consecutive test and so on. I will be sorting these by test type and will need to determine the test run length based on the start of a test and the start of the test after it. As you will note some of the test actually have a TimeTo Run feature and these tests are fine. I will be adding new tests that do not have this TTR variable and thus need to extrapolate the Test Run from start times of the test in question and the test that follows it.

For example if the test 1 began at 12:25 and the test 2 began at 17:05 then I want to know that the test run took 4 hours and 40 minutes.

Thanks
Shaun