1 July 1968
South Africa was one of the countries that signed the Nuclear Non-Proliferation Treaty to curb the spread of nuclear weapons. Other countries that signed the treaty included Britain, the Soviet Union, the USA and fifty-seven non-nuclear nations. The treaty was opened for signature on this date in New York, USA. Ireland proposed the treaty, while Finland became the first country to sign it. Click here to read the Treaty  
References

Burger, D. (ed)(2002). South Africa Yearbook 2001/02, Pretoria: GCIS, p. 222