Taken this is SQL Server you can use DATEDIFF and DATEADD in your query. Consider the following example
select DATEADD( QUARTER, DATEDIFF( QUARTER, 0, GETDATE()) - 1, 0) AS StartDate,
DATEADD( QUARTER, DATEDIFF( QUARTER, 0, GETDATE()), 0) - 1 AS EndDate
So in your where condition it could be something like the following if the quarter is precalculated into the data
SELECT ...
FROM ...
WHERE DateField = DATEADD( QUARTER, DATEDIFF( QUARTER, 0, GETDATE()) - 1, 0)
...
Or following if you just have different dates in the data
SELECT ...
FROM ...
WHERE DateField BETWEEN DATEADD( QUARTER, DATEDIFF( QUARTER, 0, GETDATE()) - 1, 0)
AND DATEADD( QUARTER, DATEDIFF( QUARTER, 0, GETDATE()), 0) - 1
...