> [!tldr] A quick guide for a frustrating topic **Excel** is notorious for being finicky with dates. How Microsoft chose to implement dates **is sensible**, but different from how dates are represented in things like [[CSV]]s... and when CSVs and Excel disagree, that's problematic. > [!warning] Spreadsheets do not like [[ISO 8601]] > Although you can make them work → [[ISO 8601 and Excel]] [[Spreadsheet|Excel]] (and Google Sheets, etc) store dates as a **decimal number of days since midnight, December 30th, 1899**. If you put `numbers` into Excel cells then format them as a ==date==. - `0` becomes `12/30/1899 0:00:00` - `1` becomes `12/31/1899 0:00:00` - `-1` becomes `12/29/1899 0:00:00` - `0.5` becomes `12/30/1899 12:00:00` Excel's eagerness to treat things as dates is the butt of many jokes: ![[Pasted image 20260328161641.png]] ![[Pasted image 20260328161910.png]] **Fun fact** I heard somewhere - some particular gene or something like that had to have its **scientific name changed** because its original sequence was interpreted by Excel as a date, which was breaking things. **** # More ## Source - self - https://www.reddit.com/r/ProgrammerHumor/comments/ulttdr/i_excel_at_optimism/ - https://programmerhumor.io/microsoft-memes/the-venn-diagram-of-misinterpreted-dates-e03m