How to set default number formatting in Numbers?

In Numbers, I've seen how to change the default cell colour and font, based on table styles.

But how can one change the default number formatting? The default "American" formatting for dates, for example, drives me nuts, and it's often the first thing I change in every new spreadsheet I create!

I would like dates to automagically take on YYYY-MM-DD format — especially if I enter them that way! But currently, they revert to DD mon YY format.

You do not change it in Numbers.

You change the the system date format from the Language & input system Prefpane. Change the Region to United Kingdom.

Next time you launch Numbers the format will be as you want.

You can use the Custmize buttons to set the look of the dates