The date/time field for GPS systems was originally a 10-bit number denoting the number of WEEKS from the start, so... the GPS week started January 6, 1980 and it became zero for the first time midnight August 21, 1999. Since that was before many devices were in the hands of the public it was just a matter of resetting the devices manually... or something like that.
Anyway, being another 1024 weeks since the August 1999 reset, the GPS Week Number was to reset to zero at midnight April 6, 2019. To push the issue off until such time as the current GPS system may be redundant and/or replaced, the 10-bit has been changed to a 13-bit Week Number. So the next reset will be 157 years from now... and thus, not our problem any more.
Ok, so what does this all mean to us?
While the huge majority of GPS-reliant devices have already been updated to use the 13-bit Week Number (phones, etc.), if you have not kept the firmware of your device up-to-date, you may experience inaccurate time and/or location.
For most of the Garmin handhelds, connect your GPS to your computer while running BaseCamp and if there is an update for your GPS BaseCamp will ask if you wish to update your device. Follow the prompts from there.
For other devices, contact the manufacturer.