class Event(models.Model):
    ...
    start = models.DateTimeField()

class SplitDateTime(forms.SplitDateTimeWidget):

    template_name = 'split_datetime.html'

    def __init__(self, attrs=None):
        widgets = (
            forms.SelectDateWidget,
            forms.TimeInput(
                attrs={'size': '5', 'placeholder': 'HH:MM', 'maxlength': '5'})
        )

        # Note that we're calling MultiWidget, not SplitDateTimeWidget, because
        # we want to define widgets.
        forms.MultiWidget.__init__(self, widgets, attrs)