[Is that character called a "carat?"])
Although the Oxford American Dictionary defines a caret "...as a mark (‸, ⁁) placed below the line to indicate a proposed insertion in a printed or written text", on the American English keyboard it is the upper case 6 or ^ that is called a caret. The > character is most often referred to as the "greater than" symbol or if you are not math-oriented the right arrow Key.
The character under the tilde is a backquote. <What is a Back Quote?
> (My daughter says she gets the same override results in Excel from an apostrophe.)
Your problem in Excel is it IS math oriented and is taking 9/30 in a different context than you intended. I just experimented in Numbers and depending on the (cell format) entering 9/30 yields 30% (percentage), 3/10 (fraction), 3E-01 (scientific), and 0.3 (formula). FWIW NeoOffice Professional Edition yielded similar results. I am not sure what format caused Excel to transform 9/30 -> 9/1 but that has to be it.
Excel has got many different date
format options, and I tried all the applicable ones in Big Sur with the same 9/30 -> 9/1 results.
But here's what I think is the odd thing: In the same column of dates, all with the same formatting option, all dates entered with several versions of Excel prior to Big Sur render as expected; it's only new entries in the column that do the 9/30 -> 9/1 trick. And...
if I delete and reenter an existing entry, the new entry does the same trick.
So, the problem seems to be that Excel's coding is somehow incompatible with Big Sur in this particular respect.
As I said, though, the backquote solves my incompatibility problem...as long as the one I've run into is my only one.