def __calculate_layover_time(arrival_time=None, departure_time=None):
    """
    This function calculates layover time between flights

    :return: (str) Layover time in hour and minutes
    """
    dtime = departure_time
    atime = arrival_time
    flight_time_sec = dtime - atime
    minutes, seconds = divmod(flight_time_sec, 60)
    hours, minutes = divmod(minutes, 60)
    total_flight_time = str(hours) + 'h' + ' ' + str(minutes) + 'm'
    return total_flight_time
