Which was the last palindromic date?

It depends how you format the date. If you do mm/dd/ format

mm = month

dd = day

= year

then the last palindromic date was September 10th, 2019 since this would be written as 9/10/2019 in the mm/dd/ format

Taking out the slash symbols, we have the number 9102019 which is the same when we reverse the digits.

The answer will be different if you have the date format as dd/mm/

its a straight line parallel to the time axis