Does the validity starts from date of entry or dates mentioned in visa

I have applied for Schengen visa and I am supposing to get visa stamp this weekend. Below is my question, kindly guide me.
I mentioned intended date of travel for visa from 03 March 2014 to 22 March 2014 for 20 days visit visa. Now I want some slight changes in flight plans due expensive flight prices.

I want to fly on 06 March 2014 and return on 22 March 2014. Is it possible?

Does the validity starts from your date of entry or its valid only between the dates mentioned in the visa.

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