python sort OrderedDict keys chronologically

I have the following OrderedDict:

 from collections import OrderedDict
 a = OrderedDict()
 a['2016:April'] = 1
 a['2016:January'] = 2
 a['2017:February'] = 3
 a['2015:November'] = 4

I would like to sort the dictionary by the keys in chronological order so that the result is:

 OrderedDict([('2015:November', 4), ('2016:January', 2), ('2016:April', 1), ('2017:February', 3)])