I checked to see that the cells were indeed dates:
(select column > format cells > number >date). That worked for some of the lines.
Some of the dates had a leading space or some other non-alphanumeric character, that happens sometimes when you’re copying and pasting data from different files. That’s easy to solve as well:
Select the date cells with a problem.
Go to Find and Replace (CTRL+F)>Replace and remove the space
That left several cells with a problem. Couldn’t figure it out, this was the brain dead moment. Spent 15 minutes on 15 or 16 lines in which I couldn’t figure out the problem.
I tried retyping the date and – lo and behold – it worked. Same thing with HTML code sometimes retyping simply works.