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David P wrote

He took off from NZ at 1150 NZ time 28^th March 1931 or 23.50 GMT on the27th.  He took off from Norfolk Is at 10.50 on 1^st April or 22.50 GMT on 31^st March.  I’ve seen other values of the GMT/NZ time difference around this date printed on the www, but I think it’s safe to assume Chichester got the time right.

Can I be bold and suggest that Chichester got the time wrong? If he did he can be excused because his flight began no more than two weeks after NZ switched from Summer Time  to Standard Time. That was before the internet and instant communications so most of the country may have been confused/ignorant of the change. Even the top scientists of the day would not have heard of NTP! I believe tjhat the BBC introduced time pips in 1924 but I do not know when they were first broadcast in NZ.

I have attached a copy of the Summer Time Act 1928. In clause 2 NZST is defined as GMT + 11 1/2. Clause 3 defines Summer Time as NZST + 1/2. Clause 4 states that Summer Time will end on the third Sunday in March. The third sunday in March cannot be later than the 21st March. Quoting the above Chichester's flight began in the 28th of March. Hence official time in NZ was GMT + 11 1/2, not GMT + 12 as you have suggested.

Of course I could be wrong.

David C