from django.utils.dateformat import time_format

@register.filter
def time_range(start_time, end_time):
    """Make time ranges like 7-9 p.m."""
    if start_time is None:
        return u''
    if end_time is None or start_time == end_time:
        return time_format(start_time, 'P')
    if start_time.hour == 12 or start_time.hour == 0 \
       or end_time.hour == 12 or end_time.hour == 0:
        return u'%s - %s' % (time_format(start_time, 'P'), time_format(end_time, 'P'))
    if (start_time.hour < 12 and end_time.hour >= 12) \
       or (end_time.hour < 12 and start_time.hour >= 12):
        return u'%s - %s' % (time_format(start_time, 'P'), time_format(end_time, 'P'))
    first_part = time_format(start_time, 'P')
    first_part = first_part.replace(' a.m.', '').replace(' p.m.', '')
    return u'%s-%s' % (first_part, time_format(end_time, 'P'))