Apart of requirements that vessel shall be arrived, ready for loading and at the charterers’ disposal charterparty, often expressly provides for compliance with some specific conditions for notice of readiness to be valid. It can be required, for example, that the vessel has to be entered in the customs’ house before notice was given or free pratique has to be obtained or that the notice to be given within stipulated time window.
Per Mustill LJ in Transgrain Shipping B.V. v Global Transporte Oceanico S.A. (The Mexico 1),  1 Lloyd’s Rep. 507 at p.510 and 513:
Thus, unless something happened after the notice was sent to make the Laytime start, it never started at all, with the consequence not only that the owners have earned no demurrage, but also that they are obliged to pay the charterers despatch money for the whole of the Laytime… I confess to some difficulty in finding the necessary elements of a waiver in the bare fact that a discharge was carried out.For example, in Pteroti Compania Naviera S.A. v National Coal Board,  1 Lloyd’s Rep.245;  1 Q.B. 469,… Since, however, Counsel in the present case are at one in stating that Pteroti sheds no light on the problem now before us I say no more about it, and I am content to accept the charterers’ concession without further scrutiny, reserving the point for detailed exploration if it should arise in the future.
I would therefore agree with the learned Judge in his rejection of the argument that the notice was a delayed-action device, effective to start the Laytime automatically when, at a later date, the ship became ready to discharge the contractual cargo: and also the linked argument that time began when the charterers knew or ought to have known of the readiness.
Per Thomas J in TA Shipping Ltd v Comet Shipping Ltd, (The Agamemnon),  1 Lloyd’s Rep 675 at p.680:
A notice of readiness which is effective to start Laytime running can only be given when the conditions set out in the charterparty for its giving have been met. A notice that does not meet those conditions is not a valid notice. Although it is often correct, when considering the statement in the notice of readiness as to the actual readiness of the vessel to discharge cargo, to speak of the truth or accuracy of a statement as to readiness, that is a way of saying that that contractual condition has been met. Again when considering the condition as to the geographical position the vessel has reached, the statement in the notice of readiness may or may not be true as to the geographical position of the vessel or whether she has reached the point that is nearest to the port or berth (as the case may be); what matters, however, is whether the condition in the charter-party for the giving of notice has been met and the vessel is, at the time the notice is given, at the point stipulated in the charter-party where notice can be given. That is the test for validity.
Per Potter LJ in Flacker Shipping Ltd v Glencore Grain Ltd (The Happy Day),  EWCA Civ 1068,  2 All ER (Comm) 896,  2 Lloyd’s Rep 487 at paras 77 and 85:
77. …In my view the circumstances of the case and the demands of commercial good sense are such that the Court should be reluctant to apply or adopt doubts expressed in obiter dicta (even from so distinguished a source as Lord Justice Mustill) so as to arrive at a result whereby, despite the fact that the vessel has arrived, NOR has been tendered and the unloading operation commenced without any reservation expressed in respect of it, the charterers are free of any constraints upon the time which they take in unloading and, despite delays for which they would otherwise be liable for demurrage, they are in fact entitled to despatch.
85. In the context of this case I would answer the question of law in relation to which leave was granted as follows. Laytime can commence under a voyage charter-party requiring service of a notice of readiness when no valid notice of readiness has been served in circumstances where
(a) a notice of readiness valid in form is served upon the charterers or receivers as required under the charter-party prior to the arrival of the vessel;
(b) the vessel thereafter arrives and is, or is accepted to be, ready to discharge to the knowledge of the charterers;
(c) discharge thereafter commences to the order of the charterers or receivers without either having given any intimation of rejection or reservation in respect of the notice of readiness previously served or any indication that further notice of readiness is required before Laytime commences.
In such circumstances, the charterers may be deemed to have waived reliance upon the invalidity of the original notice as from the time of commencement of discharge and Laytime will commence in accordance with the regime provided for in the charter-party as if a valid notice of readiness had been served at that time. By answering the question in that way, I should not be thought to doubt that, in appropriate circumstances, the same result may follow by application of the doctrines of variation and estoppel.
Per Moore-Bick J in Triton Navigation Ltd v VITOL SA (The Nikmary),  EWHC 46 (Comm),  1 Lloyd’s Rep 151, at para 42
To give notice of readiness prematurely is not a breach of charter; it is simply ineffective to start time running. Once the vessel had been properly cleaned a valid notice of readiness could be given and was given. Only then did the charterers' obligation to load within the laydays arise …