calc auto fill date increments year, not day

Have column formatted as “Date, Default - English (USA), 12/31/1999”
First row is “5/20/2020”. When I try to autofill the column, I get 5/20/2021,5/20/2022, etc, not 5/21/2020,…
What am I doing wrong?

Your dates are texts.
Remove a probably set explicit horizontal adjustment from the respective column, and you will see the contents left aligned how it is done with text. Numbers would be shown right aligned.

The standard representation of dates in spreadsheets is by numbers counting the days starting with the 0-day you mentioned. Only dates represented this way can be incremented automatically as you want it, independent of the format chosen for the display.

BTW: The “mid-endian” date formats are used next to nowhere in the world except for the USA, and by people thinking “American must be good”, or forced by their US counterparts. Safe communication of dates requires the usage of ISO 8601 (YYYY-MM-DD).

I don’t promote the format – it’s what I’m given. (I prefer Julian dates and the metric system too, but…) The dates are coming from data published on the web, over which I have no control. Isn’t there a function that would convert to a numeric format, or generate a separate column with numeric dates that could be manipulated and plotted?

I may have failed in understanding you. Data (also if dates) reperesented by numbers you can display in any format for which a format code exists. A date displayed this way as 3/5/2020 by default in a US locale is exactly the same number which would be displayed as 5/3/2020 in a UK (or other English) locale.
If you import/insert dates from external sources (web, csv supplied by someone …), you always need to first check if they are text or numbers. In many cases they will be text, and due to te human bias towards unclear and ambiguous ways of communicating, you need to thoroughly decide how they must be interpreted. You can then use the decision during the import dialog (if any), or after the import intio a dedicated column using the tool >Data>Text to Columns....

As long as your dates are texts, the Numbers format of the respective cells doesn’t matter at all.