The world did not come to an end this past weekend when the 10-bit calendar-week counter in the Global Positioning System’s precision timing system “rolled over” back to 0000000—an event that caused older, unpatched GPS systems to suddenly act like they had jumped nearly 20 years back in time. But the long-anticipated reset of the calendar count did apparently lead to cancellations of some airline flights overseas, as technicians failed to catch warnings and update firmware.

According to reports on social media, at least one KLM flight—a Boeing 777 bound from Amsterdam to Bogota—and flights involving as many as 15 Boeing 777s and 787s in China were delayed or canceled over the weekend because of calendar-rollover errors on navigation systems aboard those aircraft. Data for some of the flights identified confirmed lengthy delays in departures, with the KLM flight leaving seven hours behind schedule.

A Reddit user reported that his girlfriend’s KLM flight, KL741 from Amsterdam’s Schiphol Airport, was “grounded because of ‘something to [do] with the date being wrong and Honeywell can't guarantee the plane is safe.’”

Honeywell provides the flight management and navigation software for the Boeing 777 and 787. The company issued two service information letters in March warning of the GPS week-counter rollover and providing technical support data.

The same issue appeared to hit a Shanghai Airlines Boeing 787 (tail number B-1113), as well as over a dozen other aircraft based on a photo posted to Twitter by China Aviation Review:

Multiple Boeing 787s in China experienced GPS 20 years rollover issue. Some aircrafts have to be grounded waiting for an update. pic.twitter.com/IEFF2GHIt2 — ChinaAviationReview (@ChinaAvReview) April 7, 2019

The flight of the aircraft shown in the photo, MU 568 from Singapore to Shanghai, was delayed by four-and-a-half hours. The date on the navigation display shows the week rollover set the calendar back to 1999. According to China Aviation Review, some other Chinese airlines’ flights were canceled as aircraft awaited software updates.