There are a few ways you can get the number of days between two dates.
Using the Calculated Columns transform
Use this formula:
If you are comparing two date fields without timestamps, then it will return a whole number. If one or both have a timestamp, then it will give you the number of days, hours, minutes, and seconds between the dates.
Using a SQL transform:
DATE_PART('day', "end_date" - "start_date") as "days_between"
"end_date"::DATE - "start_date"::DATE AS "days_between"