See: http://www.englishclub.com/grammar/prepositions-at-in-on-time.htm
– NoneNov 19 '13 at 15:41
1
You would use in, but if you added the day, the preposition would change to on: ELL opened in January 2013; the first ELL question was asked on January 23, 2014.
– J.R.Nov 19 '13 at 18:42