I wish to determine when I have a contract coming due in the next 45 days and provide a note that warns me that the payment is coming due.

You can use an 'If' formula. With this formula the Due date is in Column B. The note appears in the cell with the formula. this checks for dates 45 days or less and if 45 or less, checks if it is 10 days or less.

=IF(TODAY()-B2<=45,IF(TODAY()-B2<=10,"Due Within 10 Days!","Due Within 45 Days"),"")