What's with 12AM Friday January 01, 1904?

Question

For those of you who don't know, LabVIEW keeps track of time elapsed since 12AM January 01, 1905 in order to calculate the current date and time in seconds.  I can understand the January 1st part, and the 12AM part, since is marks the beginning of a new year, but why 1904?  Why not 1905, or 1923?  Or better yet, going with the whole beginning of a new year, why not make it the beginning of a new century also, and pick 12AM, January 01, 1900?  Has anybody ever heard of a good reason for picking this day and time to start counting seconds?
~Jo-Jo

Answer 1

Here's the deal with January 1, 1904. Most (all?) operating systems store time as the number of seconds ( or microseconds, etc.) since a certain time. This time is call the epoch. The number of seconds that the Mac could store covered about 130 years. The programmers wanted the system to be able to cover the birthdays of most users, so the epoch needed to start in the early 20th century. So why not start at January 1, 1900?
The Macintosh designers needed to squish a lot of code into a very small amount of ROM/RAM. They were looking for any kind of tweak that could save them some room. The code to turn the number of seconds since the epoch into a date (like March 29, 1973) needs to know how many days there are in a year. Which means you need code to determine if a year is a leap year.
A year is a leap year if it is a multiple of 4 unless it is a multiple of 100 except if it is also a multiple of 400. So, 1900 wasn't a leap year (multiple of 100), but 2000 was (multiple of 100, but also a multiple of 400). By avoiding the year 1900, the Mac designers never had to worry about the 100/400 part of the leap year rule (because the 130 year span would only cover 2000 which was a leap year). If the year was a multiple of 4, it was a leap year.
So why not 1901 or 1902 or 1903? By starting with a leap year they didn't need an 'offset', just another way to save a little bit of space.
And since LabVIEW started on the Mac, it inherited some of those quirks.
Today the Mac doesn't use the 1904 epoch, but LabVIEW still does. NI needs a cross-platform epoch and a standard doesn't exist so January 1, 1904 is as good as any.
Pat