GPS Week Number Rollover
Ticket terminal in Prague, showing August 23 as the date (instead of April 8) after the 2019 GPS week number rollover
The Global Positioning System (GPS) Week Number Rollover is a phenomenon that happens every 1024 weeks, which is 19.7 years. The Global Positioning system broadcasts a date, including a weekly counter which is stored in only ten binary digits. The range is therefore 0–1023. After 1023 the internal value "rolls over", changing to zero again. Software which is not coded to anticipate the rollover to zero may stop working or could be moved back in time by 20 or 40 years. GPS is not only used for positioning, but also for accurate time. Time is used to accurately synchronize payment operations, broadcasters, and mobile operators.
The first rollover took place midnight (UTC) August 21 to August 22, 1999, when GPS Week 1023 advanced and rolled over to 0 within the counter.
NavCen issued an advisory prior to the rollover stating that some manufacturers have not fixed the issue. Due to relatively limited commercial and consumer use of GPS during the 1999 rollover, disruption was minor.
The second rollover occurred on the night of April 6 to April 7, 2019, when GPS Week 2047, represented as 1023 in the counter, advanced and rolled over to 0 within the counter. The United States Department of Homeland Security, the International Civil Aviation Organization, and others issued a warning about this event.
Products known to have been affected by the 2019 rollover include Honeywell's flight management and navigation software that caused delays for a KLM flight and cancellations for numerous flights in China because the technicians failed to patch the software.  Furthermore, the New York City Wireless Network (NYCWiN) crashed.  Other products that were affected by the rollover include cellphones that were sold in 2013 or earlier,  Australian Bureau of Meteorology's weather balloons,  NOAA's weather buoys,  many scientific instruments,  and consumer GPS navigation devices. 
Prior to return to normal standard time from daylight saving time during the morning of November 3, 2019, Apple issued a warning to owners of iPhone and iPad, which were sold before 2012, that these Apple products could lose internet.  iPhone 4s, iPad mini (1st generation) with cellular, iPad 2 with CDMA cellular or iPad (3rd generation) with cellular require software updates with at least iOS 9.3.6.  iPhone 5 and iPad (4th generation) with cellular require software updates with at least iOS 10.3.4.  If these devices do not have these software updates before November 3, 2019, the devices will lose over-the-air software updates and iCloud Backup. 
The above rollovers are due to a ten-bit week number; the more recent CNAV protocol, successor to the original NAV protocol, uses thirteen-bit week numbers, which amounts to a 157-year cycle; therefore, using the same epoch of 1980, the first rollover won't be until 2137.
- time formatting and storage bugs
- Structure of the time-encoding components of GPS signals, NAV and CNAV versions
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- "The April 2019 Global Positioning System (GPS) Week Number Rollover". Energy.gov. Retrieved 2019-03-14.
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- Gallagher, Sean (2019-04-09). "Somebody forgot to upgrade: Flights delayed, cancelled by GPS rollover". Ars Technica.
- Divis, Dee Ann (2019-10-17). "GPS Rollover Hamstrings New York City Wireless Network and a Handful of Other Systems". InsideGNSS.
- Shekar, Shruti (2019-04-09). "Telus contacting customers in advance about GPS rollover that may affect some customers". Mobile Syrup.
- Cozzens, Tracy (2019-04-09). "GPS Week Rollover grounds Aussie weather balloons, Boeing planes". GPS World.
- "Effects of GPS Rollover on Weather Buoys and C-MAN Stations" (PDF). NOAA.
- Butler, Declan (2019-04-03). "GPS glitch threatens thousands of scientific instruments". Nature. doi:10.1038/d41586-019-01048-2.
- Vincent, James (2019-03-08). "Older GPS devices are facing their own mini Y2K bug next month". The Verge. Retrieved 2019-08-17.
- "Apple Warns Some iPhone Users: Update your phone or lose internet". WHO-TV. Des Moines, Iowa. October 31, 2019. Retrieved October 31, 2019 – via CNN.
- "GPS.gov: GPS Week Number Rollover - April 2019". www.gps.gov. Retrieved 3 September 2020.