How to make date YYYY-MM-DD (ISO-8601) the default in Excel?

For that you actually need to change the whole Windows date format. For that, go to:

Control Panel > Regional and Language Options > Change date, time or number formats

There, choose for Short date the yyyy-MM-dd format, and done!

Here's the proof it works. Even if you write another date format, Excel will automatically convert it to the system's default.

Excel date format changing

Keep in mind that this will also change the way Windows displays the date.

Windows date format set to yyyy-MM-dd

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