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Understanding Excel Date and Time Formats
Blame It on Sisogenes
Blame It on Sisogenes
The programmer who designed Lotus 1-2-3 was not a date fanatic.
Back in 45 BC, an astronomer named Sisogenes calculated that the
earth took 365 days, 5 hours, 48 minutes, and 46 seconds to travel
around the sun. He advised Julius Caesar that this was close
enough to 365.25 days, and the leap year was born.
This worked through Caesar s lifetime. But those missing 11 minutes
and 14 seconds began to add up. By 1582, things were out of whack by
about 11 days. The spring equinox was falling on March 10 instead of
March 21.
Pope Gregory mandated that the calendar jump by 11 days. In Cathol-
ic countries, they went from October 4, 1582 to October 15, 1582. Oth-
er countries, though, resisted the change. England finally added the 11
days in 1752. Russia added them in 1918. Historians note that there was
rioting over the change (possibly from all the people who lost out on
their birthday cake?).
To prevent further rioting, Gregory proposed that we skip three leap
years out of every 400 years. This led to some arcane rules for leap
years:
Leap years happen in years divisible by 4.
Leap years are skipped if the year is divisible by 100.
Leap years are not skipped if the year is divisible by 400.
The date February 29, 2000 was actually an exception to an exception
to an exception to an exception. But everyone thought it was just an-
other leap year.
The problem is that there was no leap year in 1900. The programmer
working on Lotus 1-2-3 in Mitch Kapor s Cambridge basement didn t
know this rule and programmed a 2/29/1900 into Lotus 1-2-3.
By the late 1980s, there were millions of Lotus spreadsheets created
that had dates in them. Any competitor to Lotus had to ensure that its
program would come up with the exact same result as the industry-
standard Lotus 1-2-3. This forced Excel, Quattro, and others to pro-
gram the same error into their packages. Now, billions of spread-
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