A New Year’s time zone puzzle

A friend of mine got on a plane last night in San Francisco, at about 10:30 pm local time. She’s landing in Sydney at about 8:30 am local time on January 1. She asked: Does she get a New Year (i. e. is her local time ever midnight on December 31 / January 1)?

As far as I can tell, the answer is no. Here’s a map of the flight. The locations of the airports in question are:
SFO (37°37’08″N 122°22’31″W)
SYD (33°56’46″S 151°10’38″E)
The answer I gave was as follows: approximately speaking, she crosses over the date line about two-thirds of the way through the flight (since SFO is roughly 60 degrees east of the line and SYD is roughly 30 degrees west of it). It’s a fifteen-hour flight, so that’s ten hours in. At that time it’ll be 8:30 AM Dec 31 in San Francisco (UTC-8) so 4:30 AM Dec 31, UTC-12. At that time she crosses the line to 4:30 AM Jan 01, UTC+12 (which is 3:30 AM in Sydney, which is UTC+11 this time of year). So the answer to the question is yes.

(Note that I made the crude assumption that the date line is the 180th meridian, which it isn’t exactly. But the answer isn’t even close to depending on that.)

The flight in question is, I believe, United 863, which as of this writing hasn’t crosses the date line yet, but you can look at the tracking log for yesterday. The flight that left on the night of the 29th crossed the date line just around 11 AM US Eastern time (UTC-5) i. e. 4 AM UTC-12/UTC+12. The flight that left on the night of the 30th made that crossing around 12:41 PM US Eastern time, i. e. 5:41 AM at the 180th meridian. (The flight appears to have been closer to a great circle yesterday but took a more southerly route today, based on the FlightAware tracking pages here and here.) Coincidentally this is about when this post is going up.

In other Pacific Ocean time zone fun, there was no such day as December 31, 1844 in the Philippines. Their trade links had previously come from the east (the Americas) but as of that time had started coming more from the west (Asia).

One thought on “A New Year’s time zone puzzle

  1. Even more recently, Samoa skipped a day just a couple years ago as it did the same transition (moving from the east side of the international date line to the west side).

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