When does H1B 6Yr validity start, after stamping or the first time you enter USA?

Dear Readers,

I have done my H1B visa stamping on january 2015, and my petition approval date is Oct2014
My question is When does H1B 6Yr validity start, after stamping
or the first time you enter USA? Please respond

As per my understanding it starts from the time you Enter USA.

It will start from your arrival day to US.