def duration_str(t1, t2):
    """String difference between pendulum datetimes."""
    delta = t2 - t1
    d = delta.days
    h = delta.hours
    m = delta.minutes
    if d > 0:
        result = '%dd-%d:%02d' % (d, h, m)
    else:
        result = '%d:%02d' % (h, m)
    return result
