<p>What's the most efficient way to calculate the last day of the prior quarter?</p>

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

<p>Platform is SQL Server </p>

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.