def elapsed_time_from(start_time):
    """calculate time delta from latched time and current time"""
    time_then = make_time(start_time)
    time_now = datetime.utcnow().replace(microsecond=0)
    if time_then is None:
        return
    delta_t = time_now - time_then
    return delta_t
calculate time delta from latched time and current time