Excel: A macro to minus a certain number from a column of cells every 24 hours?

I have two columns. The first has 'Usage per day' and the other has 'Amount in stock'. I need a macro that would take the usage per day from the amount in stock every 24 hours so that I will know when I need to re-stock. I think it would need have a separate 'run' button for each cell because I would not be restocking each shelf at the same time so resetting each cell simultaneously may not work. Where would I find out how to make something like this (for a begier excel user)? Thanks

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