Counting back from the last frost
You enter a last-spring-frost date and the calendar anchors on it. For every crop on your list it subtracts that crop's standard indoor-start lead time - commonly 4 to 10 weeks - to get a sowing date, then applies a second offset for transplanting: cool-season crops are set out before the anchor, warm-season crops at or after it. Move the frost date and every date shifts with it.