Then your nukes are illegal.
Exactly. This is the point.
If the date set in NPT happened to be 1960 rather than 1970 .... there would have been only four big countries with "legal" nuclear weapons.
Would China have considered it's own weapons "illegal" too?
China had no role is setting the date to be 1960 or 1970.
Would China still have held the NPT as a Pillar of High Non-proliferation standards, if it were asked to sign the NPT as a Non-Nuclear Weapon State?
In fact, I also wonder .... will China have ever joined United Nations if it were not offered a permanent member of UN Security Council?
I think China's approach would have been:
A. Better to be outside UN, that being a less equal (than USA or anybody else).
B. Better be outside NPT than being a less equal (than NWS).
Interestingly, India has done similar to what China would have done in case of (B), but not in case of (A).
Though, it's trying to sort out (A) from within UN.
That is one reason, it has in reality already achieved a de-facto (B).
(A) would not be achieved as easily as (B).