NavList:

Very interesting question, Frank.  As a 19th century author wrote, there is no "International" Date Line, in fact.

 

All the landlubber's IDL concept did was confuse me, until I sat down while learning CN, thought carefully about dates/time customs and conventions, and prepared that page.  Until then I had the (probably common) misconception that the date changed only once going around the world, at the IDL.  But of course it also changes wherever midnight is located.  The IDL is merely another boundary among the world's 25 time zones, with the special property that the date always changes crossing it, whereas the other time zone boundary where date changes varies with position of the sun in the sky.  Better to think in terms of time zones generally.

 

In the days of sail the IDL was a practical concept.   But the situation is much different in this jet/digital age, where a person's position changes globally in hours; where a person's correspondents could be in 5 different time zones during an Internet chat; where local communities are free to adjust their time zone boundaries to suit themselves; and where a community is free to set its own customs for daylight saving time.  So it would be wise, I think, to relegate the concept of the IDL to antiquity, and think instead in terms of using an up-to-date conversion website to find out the local time for a particular community.

 

However I think the Nautical Date Line (NDL) concept is still essential for solving CN time-date questions when reducing sights, or when sailing in the vicinity of the IDL (in my dreams, sigh), because of that special property of always changing the date crossing it.

 

Jim Thompson