Under a time-charter contract the vessel is to be delivered by the owners to the charterers for their commercial use within stipulated period of time. At the end of this time, subject to any express provision contained in the contract, the charterers obliged to redeliver the vessel to the owners.
Per Lord Denning, M.R. in Alma Shipping Corporation of Monrovia v Mantovani (The Dione),  1 Lloyd’s Rep. 115 at p.117:
(d) If the charterer sends the vessel on a legitimate last voyage - that is, a voyage which it is reasonably expected will be completed by the end of the charter period, the shipowner must obey the directions. If the vessel is afterwards delayed by matters for which neither party is responsible, the charter is presumed to continue in operation until the end of that voyage, even though it extends beyond the charter period. The hire is payable at the charter rate until redelivery, even though the market rate may have gone up or down, see Timber Shipping Co. S.A. v London & Overseas Freighters Ltd.,  AC 1;  1 Lloyd’s Rep. 523.
(e) If the charterer sends the vessel on an illegitimate last voyage - that is, a voyage which it cannot be expected to complete within the charter period, then the shipowner is entitled to refuse that direction and call for another direction for a legitimate last voyage. If the charterer refuses to give it, the shipowner can accept his conduct as a breach going to the root of the contract, fix a fresh charter for the vessel, and sue for damages. If the shipowner accepts the direction and goes on the illegitimate last voyage, he is entitled to be paid - for the excess period - at the current market rate, and not at the charter rate, see Meyer v Sanderson, (1916) 32 T.L.R. 428. The hire will be payable at the charter rate up to the end of the charter period, and at the current market rate for the excess period thereafter.