Calculating number of days Calculation formula

Hi Folks,
I don't know (can't think straight) how to create an efficient formula to calculate the numbers that appear on the Excel spreadsheet that I have uploaded. I will need this for far more than the number of students I have provided in this example.

The concept is that I need to know how to write the formula to calculate the number of days between tests for each student to appear in the cells in the column like I have typed out on the example.

My simple formula would be =B4-B5 and to keep copying down but that does not really work.

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