NPT came into force in 1970 and prevented signatories from assisting states not possessing nuclear explosives in obtaining or producing them.Article IX (3) of the treaty defined a nuclear weapon state as one which had manufactured and exploded a nuclear weapon or other nuclear explosive device prior to January 1, 1967.
This meant those who possessed nuclear weapons before January 1, 1967 — the US, UK, Soviet Union, China and France— could stock up N-arsenal without violating the NPT.
This divided the world into nuclear have and have-nots. It also widened the gap, preventing horizontal proliferation of N-weapons but legitimizing vertical proliferation by the haves. India argued this was discriminatory and stayed out of it.India now wants to be recognized as a nuclear weapon power. But for that the treaty has to be amended, particularly the ’67 cutoff date that defines a nuclear weapon power.