How can I pass a time string in the uri path to a Google Cloud Endpoints API?

I am trying to pass a time value as a string in the path to a Google endpoints API.

If I use "15:00" as the value, I get the error:

"No endpoint found for path: conference/v1/dislikedSessions/Keynote/15:00"

The path is found if I send a string like "void" or a number like "1500".

Resource Container

SESS_TYPETIME_REQUEST = endpoints.ResourceContainer(
    message_types.VoidMessage,
    typeOfSession=messages.StringField(1),
    startTime=messages.StringField(2),
)

Endpoint code:

    @endpoints.method(SESS_TYPETIME_REQUEST, SessionForms,
        path='dislikedSessions/{typeOfSession}/{startTime}',
        http_method='GET', name='dislikedSessions')
    def dislikedSessions(self, request):
      """Return sessions that are not of specified type and earlier than start time."""
      filterTime = datetime.strptime(urllib2.unquote(request.startTime), "%H:%M").time()

Yeah, I can use just the numbers (with a change to the strptime method), but I can't understand why the path won't pick up the string like 15:59?

Thanks, Justin.