Although two decimal digits can represent no more than 100 years, any 100-year span could be chosen. For example, a program could be designed to interpret the years from 30 through 99 as 1930 through 1999, yet see 00 through 29 as 2000 through 2029. In this way the program might be able to survive the millennial crisis, or at least postpone it for a few decades. In particular, anyone whose birthdate is before 1930 might find the software distinctly unfriendly. No matter how the dates are shifted or rearranged, a calendrical system that covers only a finite period is bound to bump up against a limit at some point.
Computations done with six-digit dates already cause occasional bewilderment.
There are also computer systems whose built-in time bombs have a fuse shorter than 100 years. The satellites of the Global Positioning System keep track of the date by counting the weeks since January 6, 1980. The count is maintained as a 10-bit value, and thus it has a maximum range of 1,024 weeks. Even systems that will outlast the 1900s will not get very far into the new millennium. The clock runs out on the Unix operating system in 2038, on the Macintosh in 2040 and on ms-dos in 2048.