The year 10,000 problem is the class of all potential time formatting and storage bugs that would emerge when the need to express years with five digits arises. The problem can have discernible effects today, but is also sometimes mentioned for humorous purposes.
Practical relevance
Five-digit years are already a problem today for many forward-looking analysis programs.
Examples
This problem can be seen in the spreadsheet program Microsoft Excel through at least its Office 2013 release, which stores dates as the number of days since 31 December 1899 with a fictional leap day in 1900; similarly, Microsoft Access stores dates as the number of days since 30 December 1899. In either application, a date value of 2958465 will be correctly formatted as "31 December 9999", but adding 1 to that to step over to the expected date of "1 January 10000" will cause a formatting error; in Excel, for example, it will be displayed in the cell as a series of # characters. Excel also cannot automatically convert date-formatted strings such as "12/12/2007" to dates if the year exceeds 9999; "12/12/9999" is automatically converted to a date when entered into a cell, but "12/12/10000" is not. The Long Now Foundation ran into this limitation of Excel during the design of the 10,000 year clock. In Microsoft SQL Server, the Transact-SQL API has a "datetime" type which cannot store dates past 31 December 9999. The first date is January 1, 1753. Attempting to create new datetime values past the year 9999 will result in an out-of-range error. SAP NetWeaver handles date variables as strings of 8 characters. The open source OpenOffice.org Calc program is able to display dates beyond the year 9999 correctly with five digit years, but at least through version 2.4 falls victim to the Year 32,768 problem: "31 December 32,767" is the highest available date it can properly display. 32767, or 215 − 1, is the highest positive number that can be represented using a 16-bit signed integer, adding one to this value causes it to overflow, and Calc interprets the year as a large negative number, "1 January −32,768". The GNUFortrancompiler, g77, makes reference in runtime environment limits to year 10,000 problems when using intrinsic functions with this compiler suite. The problem is simply stated as, "Most intrinsics returning, or computing values based on, date information are prone to Year-10,000 problems, due to supporting only 4 digits for the year." The failure mode suggested in all of the intrinsic functions is that, "Programs making use of this intrinsic might not be Year 10000 compliant. For example, the date might appear, to such programs, to wrap around as of the Year 10000." Python's datetime module explicitly only supports one- to four-digit dates. Using a date containing a later or earlier year results in a ValueError being raised. PHP's DateTime class can handle five-digit years, except for parsing date strings with e.g. new DateTime. Microsoft Windows becomes unstable when the Year 10000 is reached. After a restart, the date will be reset to ~1900 or somewhere near that. This was demonstrated by some YouTubers.
Unlike the Year 2000 problem, where significant digits were omitted from the stored values of years, fixing the Year 10,000 problem does not require updating old records, since all four significant digits are present. It only requires that record storage in decimal be able to store five or more digits. There is, however, a potential problem with record sets that make use of lexical sorting. For example, representations of dates in the range 10,000–19,999 would appear interlaced with dates in the range 1000–1999 rather than after the year 9999.
Mitigation
The Long Now Foundation is attempting to foster the custom of writing years with five digits, so that the year would be written as "0". This would preempt the Year 10,000 problem, but would in turn be susceptible to the "Year 100,000 problem". The InternetKermit Service Daemon uses a five-digit field for the year in the Database Record Format: "Date-time fields are right-adjusted within a field of 18 with the leading blank reserved for Y10K". ISO 8601 specifies that years be written with four digits, but allows for extension to five or more digits, with prior agreement between the parties exchanging the information.
