Logging a value that changes once a day in Google Sheets [duplicate]

In my Google Sheet cell D12 gets the difference of two numbers with =D9 - D4. How do I store the value in cell D12 to a log in another sheet on a daily basis? I would like each row in the log sheet to show the date and the difference. An onEdit() based change tracker will not work for this, because the difference should be save once a day rather than every time the sheet is edited, and because the value I want to track is in a formula cell that is never edited.