How to Calculate the Difference Between Two Dates in T-SQL

Problem:

You have two columns of the date type and you want to calculate the difference between them.

Example:

In the travel table, there are three columns: id, departure, and arrival. You'd like to calculate the difference between arrival and departure, or the number of days from arrival to departure inclusively.

The travel table looks like this:

Solution:

SELECT
  id,
  departure,
  arrival,
  DATEDIFF(day, departure, arrival) AS date_difference,
  DATEDIFF(day, departure, arrival) + 1 AS days_inclusive
FROM travel;

The result is:

Discussion:

To find the difference between dates, use the DATEDIFF(datepart, startdate, enddate) function. The datepart argument defines the part of the date/datetime in which you'd like to express the difference. Its value can be year, quarter, month, day, minute, etc. In this example, you want to express the difference in days, so the first argument is day. The two other arguments are the date from which you start counting and the date when you stop counting – In this case, departure and arrival, respectively.

In most cases, what you really want is the number of days from the first date to the second date inclusively. Then you need to add 1 day to the difference in days: DATEDIFF(day, departure, arrival) + 1.