Trying to find the Date Difference using the Multi-Row Formula

[jeprime]

I am struggling to calculate the dates of service between two dates. While I believe there is something simple I am missing, it evades my comprehension. I am using the multi-row formula to attempt to calculate the datetime differences between two dates adjacent in the same column. My results is always null, and I cannot understand why. I even tried to just make a simple calculation for an end date using the formula function and then calculating the difference using the multi-formula tool - still results in null. Any help?

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[jeprime]

I would use a data cleansing tool after the multi-row formula. It would change the nulls to zeros - see the attached image. 

[LiliR]

Thank you. It worked!