Hello @Steph Derbyshire ,
Please follow steps:
1.Create an Excel in SharePoint Online Document Library
2.On the Excel workbook, create two columns.
- Set the first column cell to Date format
- In second column cell, define the function formula: =(DAYS360(A2:A4,TODAY())&"")
3.After performing the operation, you will see that Excel will automatically calculate days between current Date

Thanks,
Echo Du
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