What is correct on 1990 or in 1990?

If you are talking about a year, it would be in 1990. If you were talking about what score a person had in a competition, you could say that they were on 1990. So it depends on the context.

In relation to a date specific month then you would refer to the it as "in" - "in march 1990"

to a specific day then you refer to "on" - "on 1st January 1990"