Change date format for column of existing non-US format datetimes in Numbers

I suspect your date is simply not in a format Numbers natively recognizes. To test this failure-mode and see the same symptom, try typing in something else it may not recognize, like 999999999999, and set it to a date format. Again, it doesn't change, because Numbers doesn't want to assume how to parse it, and risk outputting incorrect values.

A solution is to pre-parse the input by field. Here's a demo using MID to extract the year, month, and date fields by their position in the string, and hand them off to the DATE function.

=DATE(MID($A2,7,4),MID($A2,4,2),MID($A2,1,2))