Calculate the last day of the prior quarter

Question

What\'s the most efficient way to calculate the last day of the prior quarter?

Example: given the date 11/19/2008, I want to return 9/30/2008.

Platform is S

Answer 1

I came up with this (tested for all months):

select dateadd(dd,-1,dateadd(qq,datediff(qq,0,'11/19/2008'),0)),
       dateadd(dd,-1,dateadd(qq,datediff(qq,0,'10/19/2008'),0)),
       dateadd(dd,-1,dateadd(qq,datediff(qq,0,'12/19/2008'),0))

It might turn out to be the simplest.