import datetime

def encavallament(t1_ini, t1_fi, t2_ini, t2_fi):
    t_ini = max(t1_ini, t2_ini)
    t_fi = min(t1_fi, t2_fi)
    if t_ini < t_fi:
        avui = datetime.datetime.today()
        return ( datetime.datetime.combine(avui, t_fi) -
                 datetime.datetime.combine(avui, t_ini) )
    else:
        return datetime.timedelta(0)