Anyone planning to do something they very, very rarely do - something they might do once, for instance, in a blue moon - should put their plan off a month.
Because, despite reports to the contrary, including one on a major New Zealand website, a blue moon will not occur on December 31.
Instead, it will occur on January 30, at 6.18pm.
The mistake appears to have been one of forgetting to factor in differences in time zones between Europe and the United States, and New Zealand and Australia.
The report said a blue moon - the second full moon in a calendar month - would take place in December, with full moons on December 2 and December 31.
But when the Otago Daily Times checked the matter with Dunedin Astronomical Society president Peter Jaquiery, he confirmed there was only one full moon in December, but January would have two: at 7.14am on January 1, and 6.18pm on January 30.
"There is a trap," Mr Jaquiery said.
Astronomers work in Greenwich Mean Time.
But the different time zones for New Zealand meant by the time the full moon occurred, New Zealand would have slipped into a new day, and in this case, a new month and a new year.
The expression "blue moon", if reports can be believed, is said to date back to medieval England.
A work by William Barlow, the Bishop of Chichester, in 1528, contained the line: "If they say the moon is blue, we must believe that it is true."
Or not, in this case.
