Comparing Date ranges

I have two sets of dates to work with. One is an existing booking with a start and an end date. The other is a new booking with a start and an end date. I want to compare them and calculate how much overlap there is. If the overlap is over a certain amount (say 4 days), then I want to flag the user.

Is there any thing I can use in terms of a SQL query to assist in this comparison? I'm relatively new to SQL so I'm not entirely sure what functions and keywords are available to me to make this comparison.BOL (Books On Line) is the best source for some date functions..they also have sample codes..

hth|||You can you the DATEDIFF(datepart, datetime1,datetime2) function to get the difference between two dates.
Take a look at
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/tsqlref/ts_fa-fz_2c1f.asp
for SQL Date Function.|||I actually ended up checking whether the start and end dates of each set were with in a certain range using a BETWEEN X AND Y syntax.