The sales tax in my area increasing January 1, 2018 . I want to to define a number or calc field based on check dates between certain date ranges but I don't want to change any of my previous data in that same field. I know I can "store" info but don't want to Four instance , all reservations checking dates between January 1, 2018 and dec 31, 2018 should be 9.25% . Checking dates between jan 1, 2019 and dec 31, 2019 should be 10.25% . But I don't want to recalculate any of my previous records. it's simple to do a search in the date fields for date ranges but how do I do it in a calculation ?