One way to do this is have a second column, B1 to B20 which responds to the date difference, e.g., in B1 =IF(A1-NOW()<30,"X","")
cells B1 to B20 will show an 'X' if the date criteria is met, (adjust the logic as required).
In the cell that shows the warning, use conditional formating.
Select: Formula Is in the Condition 1 drop-down
Enter this formula =COUNTIF($B$1:$B$20,"X")
Select a format for the cell.
The conditional formating formula counts the number of X's in column B.
If there are no X's in column B the result is zero which is evaluated as False and there is no conditional formatting.
If the formula returns a number greater than zero, it is evaluated as TRUE and the conditional formating is applied.
The 'column B' test does not have to be in column B - it could be in a hidden column or out of sight say in column AB.