Where's Clippy when you need him?!
So - just created a new sheet to test if the formatting of the cells was important.
View attachment 1559850
Formatted column A to be plain Text. Formatted Column C to Date format. As expected, cell A4 doesn't even attempt retrieve an answer and just sits there like a spanner. Cell C4 on the other hand appears to consult some mystic Precambrian calendar, removing 6 days, 6 months and 121 years for the displayed figure.
I assume that my acolyte levels of wizardry will now guide me to outputting some relevant number? I'm waaaay overthinking this - but Excel itself does not help in the slightest.