All programs and systems that incorporate date handling routines using the Gregorian Calendar. All systems that run software, including PCs, departmental servers, telephone exchanges and security systems. Customer estimates of the cost of checking, changing and testing their applications range from five to one hundred work-years. The US Social Security Department is reported to have estimated a need for 300 work-years to resolve the problems in their applications. Though the changes needed for any one program are usually straightforward, every program needs to be checked, and if it is changed, also needs to be tested.
Further difficulties arise because of the age of some production programs.
Programmable computers have been available since the 1960s. Many programs have been running reliably for years, well beyond their expected life span, and may not have required maintenance for years. This considerably increases the costs of assessing the ability of these programs to handle dates in and beyond the year 2000. Data could be overwritten when programs write to these files. If the file naming convention is changed, file names may become invalid.