ActiveSupport >= 3 has

Date.parse('2011-02-10').beginning_of_quarter #=> 2011-01-01
Date.parse('2011-02-10').end_of_quarter #=> 2011-03-31

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You can manually calculate the quarter index like

(Date.parse('2011-02-10').month / 3.0).ceil #=> 1

Yes, you do actually divide by 3.0, not 4.0.

MySQL has

SELECT QUARTER('2011-02-10'); #=> 1