Sale of land but not declared in income tax return

Dear Sir/ Ma'am,

I have sold a land in A.Y. 2009-2010 the sale was on the basis of agreement and the same is not registered, the above sale was not included in ITR for AY 2009-10.

now my question is if i register sale of land now than what are the tax implication on me?

Is it better to show the sale in A.y. 2009-10 when agreement executed or in the current year when the same is registered.

for the purpose of valuation which stamp duty value of which year shall be considered 
: -

a) Year in which agreement executed AY 2009-10 or

b) Year in which sale registered AY 2017-18.