You can change the date & time format in Google Sheets & Excel Online using this guide. Use different date & time formats in spreadsheets.

Using specific date and time formats, you can display a date or time in many ways. That’s not how Excel stores a date, though—not as the formatted string that you see.

Trying to use an Excel Date and Time function, but Excel returns a serial number instead of date; this is because Excel stores dates and times in serial numbers in the backend; for instance, when ...