GPS’s legacy NAV message stores the week number in a 10-bit counter, allowing values only from 0 to 1,023 before resetting to zero every 1,024 weeks—roughly 19.6 years. Software that fails to account for the reset can interpret the date as approximately 20 years earlier, disrupting GPS positioning and the precise timing used by payment systems, broadcasters, mobile operators, aviation, and scientific equipment. The first rollover occurred on August 21–22, 1999, with limited disruption reported.
The second rollover occurred on April 6–7, 2019. Honeywell flight-management and navigation software problems contributed to delays involving a KLM flight and cancellations of numerous flights in China after technicians failed to apply patches. New York City’s Wireless Network for municipal services also crashed, while older cellphones, weather systems, scientific instruments, consumer navigation devices, and other equipment experienced failures or incorrect dates. Apple warned in 2019 that some iPhones and iPads sold before 2012 required updates to preserve connectivity. The next legacy rollover is expected on November 20–21, 2038; newer CNAV signals use 13-bit counters and extend the cycle to about 157 years.