Let's assume your dates are in column A, with header in A2.
On fly, into some column in row 2 enter the formula like:
Code:
=IIF(MONTH(DATE(YEAR($A2),MONTH($A2),1) + WEEKDAY(DATE(YEAR($A2),MONTH($A2),1),2)-1) + 21) = MONTH($A2),DATE(YEAR($A2),MONTH($A2),1) + WEEKDAY(DATE(YEAR($A2),MONTH($A2),1),2)-1) + 1,DATE(YEAR($A2),MONTH($A2),1) + (WEEKDAY(DATE(YEAR($A2),MONTH($A2),1),2)-1) + 18)
, and copy it down.
Alternatively you can define some dynamic Name, and use it in same formula, like:
activate any cell in row 2;
Define Name e.g.
Code:
nFirstMonday = DATE(YEAR($A2),MONTH($A2),1) + (WEEKDAY(DATE(YEAR($A2),MONTH($A2),1),2)-1)
Your formula will be now like
Code:
=IIF(MONTH(nFirstMonday + 21) = MONTH(MONTH($A2), nFirstMonday + 21, nFirstMonday + 18)
Now you can use this calculated column as source for conditional formatting, for filtering data, or for whatever.