The number of days between the last frost of spring and the first frost of fall is known as

It is known as growing season.  To measure the length of your growing season you should count the day number between the average last-frost date in spring and the first-frost date in fall (your Cooperative Extension Office or a local nursery can give you these dates). Last-frost dates teaches you to  know the safest to set out tender vegetable plants or sow seeds.