The movie was based in the years 1977-1980 judging by Saturday Night Fever, Star Wars, etc. In those days US$1 was approximately 8-9 Indian Rupees. So in the beginning when they said $2 was 125 Rupees, that is based on currency rates of 2015 on wards.
Smith's mother says that he can have his Halloween costume after dinner as a Diwali present. The movie claims Diwali and Halloween were on the same day in 1979. Halloween is on October 31st, whereas Diwali was on October 20th in 1979.