Z:gnu-www-ja-year2000--c5b8b1-The main reason is theoretical/en

The main reason is theoretical. GNU systems, and Unix-like systems generally, represent a date/time as a 32-bit integer, counting seconds from the beginning of 1970. This 32-bit count will overflow in 2038; but there will be no problem in that year, because by then all systems will have redefined  to be a 64-bit integer.