How do I sort a list of datetime or date objects?

How do I sort a list of date and/or datetime objects? The accepted answer here isn't working for me:

from datetime import datetime,date,timedelta

a=[date.today(), date.today() + timedelta(days=1), date.today() - timedelta(days=1)]
print(a) # [datetime.date(2013, 1, 22), datetime.date(2013, 1, 23), datetime.date(2013, 1, 21)]
a = a.sort()
print(a) # prints 'None'....what???