B777 VNAV
.It appears if say cruising at FL350, FMA indicating VNAVPATH and ATC instructs a descent to say FL310, the descent occuring say 100nm from TOD. It appears the a/c will descend in VNAVSPEED, however on reaching FL310 the FMA will change to VNAVPATH and a new TOD will be calculated.
However apparently on some ocassions if ATC instruct you to descend early to meet a WPT constraint. The a/c will descend in VNAVPATH but on reaching the new altitude. The FMA will change to VNAVALT and there will be no new calculation of TOD. (window closed). Could somebody explain please.