Formation of a time charterparty, same as a voyage charter, governed by the ordinary rules of the law of contract, i.e. there must be an offer and acceptance, then the parties must have agreed all the essential terms either in writing or verbally to make the contract binding.
In Torvald Klaveness A/S v Arni Maritime Corpn (The Gregos),  1 Lloyd’s Rep 1 per Lord Mustill at p.7:
…if the order is apparently valid its validity is no more than contingent, since the time for matching the service against the promise to serve does not arrive until the nature of the service is definitively known; and this will not usually be until the service is due to begin, or in some instances until it is already in progress. Thus, if and for so long as the service required conforms with those which the shipowner promised in advance to render the specific order creates a specific obligation to perform them when the time arrives. But only for so long as that state of affairs persists. If circumstances change, so that compliance with the order will call for a service which in the original contract the shipowner never undertook, the obligation to comply must fall away. As I see it, the charterer’s order in advance amounts to a continuing requirement, the validity of which may change with the passage of time.