Hi,

Try this:

ColA: your 23.000 values starting in A3
ColB: the formula below
Row1: validation list (the dates)

=ISERROR(VLOOKUP(A3,$A$1:$F$1,1,0)) --- False or True.
Copy down to B23000. Use Autofilter to find all True.

Ola


To avoid 'fat-fingers', try Input Validation:
This will mean that you only Allow any of your 6 dates.

1. Select all your 23.000 values
2. Goto menu: Data/Validation/
3. Select: Tab; Validation, Allow; List
4. Then select your range with the 6 dates, hit Enter.