Help! If cell contains any sort of date, show green checkmark. Otherwise, show red...

we can do that if the icons are in the next column and not in the same column as the date
and you can choose show ICON only

in B1 put
=IF(A1="",3,1)
Now copy down the column B

set the icons to
>=2.1
>=1.1
and reverse the icons and click on only show icon

this will not test for a date - anything in the cell other than a blank will be a tick

i thought maybe in column b

=IF(AND(CELL("format",A1)="d1",A1>32874,A1<55153),1,-1) that will cover text and any date between
01/01/1990
31/12/2050
then icon set rule is number
>0 red
>=0 yellow which it ignores
and <0 red
show icons only
note it only detects the cell formatted as date in a certain format so if you are using a different one you need to change "d1" to match
the formats are
m/d/yy or m/d/yy h:mm or mm/dd/yy "D4"
d-mmm-yy or dd-mmm-yy "D1"
d-mmm or dd-mmm "D2"
mmm-yy "D3"
mm/dd "D5"