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.
The only place they are off by one day takes place 120 years ago. I cannot see how the attached file interacts with that. All uses in it, etiher absolute dates, or days between dates, would be far more recent than 120 years ago.
If you use DATEDIF() in a couple of the ways it can be used, there are known difficulties. Not bugs as usually claimed, but difficulties due to how Excel help expresses the way the options should work, but actually, if one looks at them differently ("stupidly" being a synonym here, as would be "convoluted"), they become understandable and give the results you'd expect. No one has time for that though, so if you use that function, choose a new way and just avoid the whole rabbit hole involved.