Nice. Now we'll find the difference between two moments in time. SQL Server offers the useful DATEDIFF() function for this, and it looks like:

DATEDIFF(datepart, start, stop)

This function returns an integer that is the difference between the start and end (stop) dates. The datepart argument specifies the time period (minute, hour, week, year, etc). The returned value is based on the unit given in datepart. You can use DATEDIFF() to find time differences as well as date differences.

The query below returns the number of weeks between March 14, 2017 and March 30, 2017:

SELECT
  DATEDIFF(week, '2017-03-14', '2017-03-30');

This returns a 2, and there are indeed two whole weeks between those dates. However, the actual difference is 16 days, which is more than two weeks. Since we specified weeks, though, we get a two.

What if we reverse the start and stop dates?

SELECT
  DATEDIFF(week, '2017-03-30', '2017-03-14');

In this case, the function returns a -2. The negative signifies that the end date is before the start date.