The date serial numbers in excel is off by one day. This is a problem for me since I use the dates to record my blood sugar due to Type "1
David H commented
Think may be how the date format is set up to me 07/09/21 is 7th of September 2021 that will be a Tuesday.
Of if you are American you may read it as 8th of July 2021 that was a Thursday
But then if you assume the year is 21 -- day depends on what calendar you are using -- also age of the version of Excel -- some may see it as 1921 and not 2021 ?
May be give 13/08/2020 a go and see what you get -- at least that way should flush out month and day of month error due to format set in OS or over ridden by Excel or setup of individual spread sheet
I Posted that the date format was not correct. Please try it yourself to make sure in am not doing it wrong. If you use column "B" to list a date. 07/09/21. Then in column "A" enter as a conditional format =TEXT(B1,"DDD") you should get THU. However, column "A" returns FRI. It appears that the serial number is off by one day. This is possibly due to leap years having 29 days.