He was born in January, 1990.

Along with his jockey, Patrick Valenzuela, the horse Comma rode to a top spot during the 137th Kentucky Derby.

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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.

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