Wrong: He was born in January, 1990.
Why: There's a comma between the month and the year.
Right: He was born in January 1990.
Why: While month/day/year combinations do take a comma before the year, as soon as you remove the day from the equation, the comma drops away. A unit consisting only of a month and a year needs no comma to divide the elements, perhaps because there aren't two numbers side by side, which can invite confusion.
Next, another one relating to dates …