Same day

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The Newsweek story begins:

How's this for a coincidence? Charles Darwin and Abraham Lincoln were born in the same year, on the same day: Feb. 12, 1809.

(Similar stories can be found all over the place.) And that day is tomorrow, Thursday 12 February.

But wait! It already is Thursday 12 February some places — much of Australia, for instance. "Same day" here means 'same date, as determined locally'. Dates and times are reckoned locally; they are relative to a location and depend on conventions for labeling spans of time (via time zones and the like). These conventions allow us to say that Charles Darwin and Abraham Lincoln were born on the same day, 12 February, meaning that it was 12 February in Shropshire when Darwin was born there and 12 February in Kentucky when Lincoln was born there.

And now it's Thursday 12 February throughout Australia, though it wasn't when I started writing this posting. But it won't be Darwin/Lincoln Day here in California until tomorrow.

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