How did you actually want to sort your data then? If you wanted to sort in date order neither a <=> or a cmp will be appropriate for that date format.
You could try using a yyyymmdd type format or convert the date and time into an epoch time (and sort that with <=>) or look at any functionality provided by any Date modules. (I'm not really familiar with any of these, so I'd be interested to hear about any date sorting they might provide)