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A calendar back to astronomical time
The Endless Calendar
The calendar originally ran from 1904 to 2096. Then someone considered that a historical app might need to go further back, so it was reprogrammed to reach all the way back to year one and much further forward, too. Then it was realized that human history is longer than that, and it was reprogrammed again to go back to the beginning of astronomical time.
Games and Animations
The game room held an animated toss a coin function. Click the coin and wait a second, then another second while the 1994 microprocessor renders the graphics, and the coin lands on tails. Andy Hertzfeld, a co-founder of General Magic and one of the programmers behind the Macintosh, wrote it.
The device also carried a portable panel of musical instruments you could sample, and cute animations of animals where you could pick up a cartoon cat, release it, and watch it tumble and land on its feet. Big animated emojis, including a walking lemon, could be attached to messages. All of it ran on little processing power, little memory, and a short battery life.
Why is calendar math hard for software?
Leap years follow a rule: years divisible by 4 are leap years, except those divisible by 100, unless they are also divisible by 400. The Gregorian calendar adopted this rule in 1582, and countries switched at different times. Dates before a country's switch need a convention, such as extending the modern calendar backward, which is called a proleptic calendar. In 1752 Britain and its colonies dropped eleven days from September to catch up.
Why does a 1904 to 2096 range simplify code?
Between 1901 and 2099, every fourth year is a leap year, because 1900 and 2100 are not. Code limited to that window can skip the century exceptions. The classic Mac operating system counted seconds from January 1, 1904, the first leap year after 1900. A 32-bit counter of seconds reaches its limit in 2040.
Why do astronomers use a year zero?
Astronomers number the year before 1 AD as year 0, so 1 BC is year 0 and 2 BC is year minus 1. The everyday historical calendar has no year zero, which makes subtracting across the BC and AD boundary awkward. Astronomers also use Julian day numbers, a running count of days from noon on January 1, 4713 BC. Joseph Scaliger proposed that system in 1583.
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