Those who know anything about computer sort rules will know why :-)

Er...... In Excel it depends where you sort. And your settings. In cells, if you have selected UK format, it is happy to oblige. Indeed if you extend-fill a single date it will index the date correctly (to chosen format).

BUT.............

If you programme in VBA it gets confused with aa/bb/cccc so the only way to unconditionally deal with dates is to convert from expected format to cccc/bb/aa & thence to a long variable.
If you don't know what format is coming in, you have to force the in-putter to deliver the individual elements with a custom process/box/dialog.

And dates before 1970? But sometimes 1900?

Don't ya just love how computers make life simple?