This is interesting.
A long distance voyage by ship in the 1920s would have an estimated date of arrival at its destination, but not a guaranteed one: as Brian says, weather conditions etc could mean that the actual date would be one or more days either side of the estimated or timetabled date. I haven't seen any outbound passenger lists from UK which give date of arrival, as this was unknown at the outset of the voyage. Has anyone?
Also, the declaration on the "header" page of the list mentioned by Brian is about provisioning -- in other words, the shipping line was obliged by law to supply enough food, water etc for a certain number of passengers for a certain number of days. The figures are usually different to the actual number of passengers and the number of days travelling. I looked at the voyage of the Ruapeha and this says 300 statute adults (this means each adult counted as one, and each child counted as half a person!) and 49 days. The "summary" page at the end of the list shows that there were 340 "souls" on board (this excludes crew), equating to 302 statute adults. What this means, I imagine, is that the ship called in at one or more ports en route to NZ to stock up on provisions. If the ship went the direct Panama Canal route, an obvious stopping off point would have been there, but I've seen old atlases from the 1920s which show apparent stopping off points in the Caribbean (St Thomas and Jamaica). However, some ships were still taking other routes to NZ, including round Cape Horn, in which case they may have stopped off on the coast of Brazil or Argentina.
It would be great to find the actual date of arrival in NZ -- perhaps you could let us know if you find out, Brian. Good luck!