Time formatting and storage bugs

In computer science, data type limitations and software bugs can cause errors in time and date calculation or display. These are most commonly manifestations of arithmetic overflow, but can also be the result of other issues. The best-known consequence of this type is the Y2K problem, but many other milestone dates or times exist that have caused or will cause problems depending on various programming deficiencies.

On 5 January 1975, the 12-bit field that had been used for dates in the TOPS-10 operating system for DEC PDP-10 computers overflowed, in a bug known as "DATE75". The field value was calculated by taking the number of years since 1 January 1964, multiplying by 12, adding the number of months since January, multiplying by 31, and adding the number of days since the start of the month. The maximum value representable using an unsigned 12-bit integer is 2¹² − 1 = 4095, and the value of 4095 represents 4 January 1975: ${\displaystyle (((11{\text{ years since 1964}})\times 12)+0{\text{ months since January}})\times 31)+3{\text{ days since start of the month}}=4095}$

Therefore 4 January 1975 is the latest encodable date. The "DATE-75" patch pushed the last encodable date to 1 February 2052, making the overflow date 2 February 2052, by using 3 spare bits from other fields in the file system's metadata, but this sometimes caused problems with software that used those bits for its own purposes. Some software may have supported using one additional bit for the date but had issues with additional bits, which could have resulted in some bugs on 9 January 1986.

Digital Equipment Corporation's OS/8 operating system for the PDP-8 computer used a signed four-bit nibble to store the number of years since 1970, and this could only represent the years 1970 to 1977.

This was recognized when the COS-310 operating system was developed, and dates were recorded differently.

Multiple Sierra Entertainment games released for the Classic Mac OS started to freeze when running on 18 September 1993. An issue in the Mac version of Sierra's Creative Interpreter (Mac SCI) would cause the game to "lock up" when attempting to handle a delay due to a problem involving an overflow. Mac SCI would attempt to use the date to determine how long a delay should last by getting the current time in seconds since 1 January 1904, the Macintosh epoch , and dividing by 12 hours. The division was processed by the Motorola 68000 and would not occur if an overflow was detected because of the division, but the Mac SCI would continue on regardless as if the division had occurred, eventually resulting in a delay of one second being treated as a delay of 18 hours and so on. Sierra released a patch called MCDATE that resolved the problem for almost 14 years .

In Apollo Computer's Domain/OS operating system, absolute time was stored as a signed 48-bit integer representing the number of 4-microsecond units since 1 January 1980. This value overflowed on 2 November 1997, rendering unpatched systems unusable.

In the last few months before the year 2000, two other date-related milestones occurred that received less publicity than the then-impending Y2K problem.