• Use commas to set off the year when using full dates.

She was born on September 15, 1985, in Los Angeles.

• Do not use commas when using only month-and-year constructions.

Planning began in September 1995.

• Do not use 1st, 2nd, 3rd, 4th, and so on with dates.

July 21
April 2

• Use the year if referring to a date not in the current calendar year.

Rory Williams and Amy Pond had a baby in December 2011.
but
The Songs had a baby in January (of this year).

• Use the following models when referring to periods of years:

She worked from 1949 to 1961.
He worked in 1949–50. (for an academic year)
He worked in the 1950s. (for a decade)
Avoid ’50s. Do not use 1950’s.

• See also months and en dash.