On the weekend Sunday the 5th of September 2021 is a big day for Excel.
In Excel the date system starts on 1/1/1900 – that was day 1, and each day since has its own sequential number.
On Saturday September 5, 2021 (Father’s day in Australia) that number is – wait for it – see the image below.
May the fours be with you!
If you remove the formatting from a cell with a date you will see the underlying number.