… for laytime to start the owner shall accomplish approaching stage or leg of voyage charter.
Per Sir John Donaldson, M.R. in Dow Chemical B.V. v BP Tanker Co. Ltd. (The Vorras)  1 Lloyd’s Rep 579 at p.584:
The words are "72 hours, weather permitting". The essence of the owners’ argument is that this phrase means "72 hours, unless the weather prevents the vessel from loading". There would be something to be said for this if the antonym for "permitting" was "preventing". But it is not. It is "prohibiting". If the phrase is to be inverted, it reads "72 hours unless the weather prohibits loading". In my judgment the weather prohibited any vessel of this general type from loading and it is nothing to the point that owing to the presence of another vessel in the berth, the prohibition was not the operative cause which prevented the vessel from loading. I would construe "72 hours, weather permitting" as meaning "72 hours when the weather was of such a nature as to permit loading".