If your start and end date columns are Continuous, Numeric, Date format; then the formula for the duration between the two columns in days could be something like: Date Difference(:Start Date, :End Date, "day")
If your date columns are character, then you can parse the text like PMroz did above with:
Date Difference(
informat(:Start Date, "m/d/y"),
informat(:End Date, "m/d/y"),
"day")
JMP Systems Engineer, Health and Life Sciences (Pharma)