Since the header can’t be inserted in Excel Online, you can’t insert the date in the header. But any header you inserted using Excel desktop application isn’t changed or deleted. The header is just invisible in Excel Online.
If you have the Excel desktop application, you can use the Open in Excel button to open the workbook and insert a date in a header. Here’s how:
Click Open in Excel and insert the date in a header.
For news about the latest Excel Online updates, visit the Microsoft Excel blog.