Longword

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. The 32-bit date value will run out and reset on February 6, 2040 on Macs using drives formatted with HFS or HFS Plus.

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. 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.