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Apple Wiki

A longword, or doubleword, is a set of computer data, 32 bits long, that is processed as a unit. A word is half as long at 16 bits.

Date longwords[]

Early Macintosh systems support 32-bit date longwords, which count the seconds passed since January 1, 1904 (1900 was not selected because it is a century year). This value could be reset if the system undergoes a serious crash or if the clock battery dies while the power is out.[1] The 32-bit date value will run out and reset on February 6, 2040 on Macs using drives formatted with HFS or HFS Plus.[2]

Mac OS X systems start counting from January 1, 1970. Because the data is counted differently, versions prior to 10.6 will run out on January 19, 2038, a limitation inherited from Unix.[3] Starting with the Apple File System, introduced in 2017 with macOS 10.13, date values are stored as 64-bit quadwords, which support dates through July 21, 2554.[4]


  1. Why Do Older Macs Reset to 1904? by Jeff Adkins, Low End Mac. 2004-01-15.
  2. Solving Mac OS 8’s Y2K20 Bug by Josh Centers, TidBITS. 2020-01-03.
  3. Is any version of OS X/macOS vulnerable to the Year 2038 problem? by Thunderforge, Stack Exchange. 2016-09-13.
  4. Decoding the APFS file system by Kurt H. Hansen and Fergus Toolan, Norwegian Police University College. 2017-04-22. Revised 2017-06-21.

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