Dates, Macs and PCs

I had a question from a friend about dates today. She had 2 spreadsheets, both showing 1/1/2017 as a date. When she copied one to another, the dates were all wrong–they were off by 4 or more years.

Here is an example of what her problem was:

The problem is that 1900 is not a leap year, according to the standard rules:

A normal year is defined as 365 days. (Plus a quarter day which is resolved by having a leap year and adding a day (Feb 29) every four years.) A year is a leap year if:

  1. it is divisible by 4.
  2. It is not divisible by 100.
  3. It is divisible by 400 if it is a centurial year. (1900, 2000, 2100, etc.)
  4. So 1700, 1800 and 1900 were not leap years, but 2000 was.

Steve Jobs insisted on the Excel numbering system starting on 1/1/1904 to avoid this since the PC started numbering from 1/1/1900 and included 2/29/1900 as a valid date.

The PC Excel File | Options | Advanced section on Excel lets you choose Use 1904 numbering system. I assume that the opposite is true on a Mac.

See Neal deGrasse Tyson on the subject: https://www.quora.com/Why-are-years-such-as-1700-1800-1900-not-leap-years-even-though-they-are-divisible-by-4-but-1600-and-2000-are


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