When does H1B VISA validity starts??


I have a valid H1B Visa[b] from oct 2012 to oct 2015[/b]. My travel may delay be another few months. So does this mean I lost 1 year of visa validity time (till now) or  3 years validity starts only once I arrive US. I would appreciate any  comments in this regard.



I feel it starts with the dates which are mentioned on the I-797 Approval notice.