Debtor Days using DateTimeDiff

[cparsad001]

Hi Community, I hope you are all well.
I am working on a workflow and I am stuck on the final phase. I am basically trying to calculate Debtor Days using the DateTimeDiff formula. However, I am not sure how to get the correct answer. 

My ultimate objective is to calculate how many days does it take for column Bills to reach each Closing Bal and output it in Debtors day column. The first answer should be around 4 days (10+20+20+30).

I have attached a copy an example of the table I am working with.

[cparsad001]

Thanks @Peachyco , the macro is providing me with the info I am looking for. I fine-tuned it and it is working perfectly with my real workflow.
Thanks everyone for your help! Very much appreciated!