problem with dateutil

[Tom P]

I am writing a program that has to deal with various date/time formats 
and convert these into timestamps. It looks as if dateutil.parser.parse 
should be able to handle about any format, but what I get is:

datetimestr = '2012-10-22 11:22:33'
print(dateutil.parser.parse(datetimestr))
result: datetime.datetime(2012, 10, 22, 11, 22, 33)

However:
datetimestr = '2012:10:22 11:22:33'
print(dateutil.parser.parse(datetimestr))
result: datetime.datetime(2016, 2, 13, 11, 22, 33)

In other words, it's getting the date wrong when colons are used to 
separate YYYY:MM:DD. Is there a way to include this as a valid format?