@Amit... Welcome to Chandoo.org and thanks for posting your question.
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Regarding your question,
If you just want the very first quarter start date
(that is for all dates between 2007 and 2012, you just want one output - 1-oct-2007):
Then, use this formula.
Assuming your dates start in cell B2, write
=DATE(YEAR(B2),CHOOSE(MONTH(B2),1,1,1,4,4,4,7,7,7,10,10,10),1)
This will give you 1-oct-2007 as output.
If you want nearest quarter date for each date
Then use the same formula in C2 (or any other adjacent column) and drag it down.