As per Rajasthan Rules Year 2000,
3. Any Birth / Death which has not been registered within one year of its occurrence, shall be registered only on an order made by Magistrate of the first class after verifying the correctness of birth and death on the production of an affidavit made before a notary public and on payment of the prescribed late fee of Rupee One only.
from this rule, it is clear that Megistrate approves notary affidavit of parents, a birth registration can be done for later dates after 1 year of birth.
I don't know if this is the rule for all the states or not in india.
With this, I want to state that people whoe have later date registration on their birth certificate, should not worry about anything illegal.
I am just putting my point because I am one of them.
My birth year is 1975 but my registration year is 2006.
The above rule is of year 2000.