Around the year 2000, Apple wrote a press release that every Mac since 1984 would avoid the Y2K problem, and could process dates for the next several thousand years. The good news is that since t...
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As described, 10.6 was the first OS X to redefine how time is stored (as 64-bit). It would be interesting to find out how the PowerPC Snow Leopard project will handle manually setting dates via Terminal.
Also described by other comments, even El Capitan won’t allow dates to be set past 2038 via System Preferences, but this can be achieved on such systems using the date command.
An interesting sidebar however is Apple made claims Mac OS 9 Is capable of dates until the year 29,940.