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.
Hyundai Merchant Marine Co Ltd v Gesuri Chartering Co Ltd (The Peonia),  1 Lloyd’s Rep 100, per Bingham LJ at p.108:
It would seem to me … that every time charter must have a final terminal date, that is a date by which (in the absence of an exonerating cause) the charterer is contractually obliged to redeliver the vessel. Where the law implies a margin or tolerance beyond an expiry date stipulated in the charter-party, the final terminal date comes at the end of such implied extension. When the parties have agreed in the charter-party on the margin or tolerance to be allowed, the final terminal date comes at the end of such agreed period. But the nature of a time charter is that the charter is for a finite period of time and when the final terminal date arrives the charterer is contractually bound (in the absence of an exonerating cause) to redeliver the vessel to the owner. I shall hereafter use the expression "final terminal date" to mean the final contractual date for redelivery, after the expiry of any margin or tolerance which the parties may agree or the law imply.