def tcb2tdb(tcb)
      jd_tcb = gc2jd(tcb)
      return tcb - Const::L_B * (jd_tcb - Const::T_0) * 86400 + Const::TDB_0
    rescue => e
      raise
    end
def tcb2tdb(tcb)
      jd_tcb = gc2jd(tcb)
      return tcb - Const::L_B * (jd_tcb - Const::T_0) * 86400 + Const::TDB_0
    rescue => e
      raise
    end
=========================================================================
 TCB(太陽系重心座標時) -> TDB(太陽系力学時)

 * TDB = TCB - L_B * (JD_TCB - T_0) * 86400 + TDB_0

 @param:  tcb  (Time Object)
 @return: tdb  (Time Object)
=========================================================================