How to Find the Difference Between Two Datetimes in T-SQL

Problem

You have two columns of the type datetime 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 the arrival and the departure.

The travel table looks like this:

Solution 1: Difference in Seconds

SELECT
  id,
  departure,
  arrival,
  DATEDIFF(second, departure, arrival) AS difference
FROM travel;

The result is:

Discussion

To calculate the difference between the arrival and the departure in T-SQL, use the DATEDIFF(datepart, startdate, enddate) function. The datepart argument can be microsecond, second, minute, hour, day, week, month, quarter, or year. Here, you'd like to get the difference in seconds, so choose second. To get the difference in hours, choose hour; for the difference in months, choose month, etc. The startdate and the enddate arguments are the starting and the ending datetime columns, respectively (here, departure and arrival, respectively).

Solution 2: Difference in Days, Hours, Minutes, and Seconds

WITH difference_in_seconds AS (
  SELECT
    id,
    departure,
    arrival,
    DATEDIFF(SECOND, departure, arrival) AS seconds
  FROM travel
),

differences AS (
  SELECT
    id,
    departure,
    arrival,
    seconds,
    seconds % 60 AS seconds_part,
    seconds % 3600 AS minutes_part,
    seconds % (3600 * 24) AS hours_part
  FROM difference_in_seconds
)

SELECT
  id,
  departure,
  arrival,
  CONCAT(
    FLOOR(seconds / 3600 / 24), ' days ',
    FLOOR(hours_part / 3600), ' hours ',
    FLOOR(minutes_part / 60), ' minutes ',
    seconds_part, ' seconds'
  ) AS difference
FROM differences;

The result is:

Discussion

First, calculate the difference between the arrival and the departure in seconds, using the DATEDIFF() function (the first CTE, named difference_in_seconds), just as in Solution 1. Then, calculate how many seconds there are in excess of whole minutes (seconds_part) to be used later to calculate the seconds, how many seconds there are in excess of whole hours (minutes_part) to be used later to calculate the minutes, and how many seconds there are in excess of whole hours (hours_part) to be used later to calculate the hours.

To do this, use the % operator. For example, an hour has 3600 seconds, so to find how many seconds there are in minutes_part, find the remainder from the division by 3600 like this:

seconds % 3600 AS minutes_part

Similarly, there are 3600 * 24 seconds in a day, so to calculate how many seconds there are in hours_part, write:

seconds % (3600 * 24) AS hours_part

Once these remainders are calculated (in the second CTE, named differences), you can finally get the difference in days, hours, minutes, and seconds. To get the number of seconds, minutes, hours, and days, divide the number of seconds in the remainder by the corresponding number of seconds in days, hours, or minutes. For example, to find out how many minutes should be displayed, take minutes_part and divide it by 60, since there are 60 minutes in an hour. You only need the integer part from this (i.e., without the decimal part), so use the FLOOR() function like this:

FLOOR(minutes_part / 60)

Finally, you simply need to display in one string what you've calculated. To do this, use the CONCAT() function in the outer query:

CONCAT(
    FLOOR(seconds / 3600 / 24), ' days ',
    FLOOR(hours_part / 3600), ' hours ',
    FLOOR(minutes_part / 60), ' minutes ',
    seconds_part, ' seconds'
  ) AS difference

The solution presented here returns a datetime difference as a text. You can easily modify the solution to get only the numbers without any text. You can also store days, hours, minutes, and seconds in different columns:

FLOOR(seconds / 3600 / 24) AS days,
FLOOR(hours_part / 3600) AS hours,
FLOOR(minutes_part / 60) AS minutes,
seconds_part AS seconds