The Casio GW-M5610U (also called GW-M5610U-1) is a digital G-Shock watch that charges using sunlight and keeps precise time by syncing with atomic clock radio calibration signals.
I like attending work meetings on time and started looking for a wristwatch that could maintain precision in time. To my surprise, I discovered that all digital/quartz wristwatches deviate by a few seconds within a month. Mechanical watches are worse (even million dollar ones) - deviate by seconds within a day!
The only precise solutions turned out to be smart watches (that sync with NTP servers over the internet) or digital watches that sync to the atomic clock radio signals that are transmitted on most continents. Needless to say that I preferred the radio sync - anything to avoid the complexity of smartwatches and their poor battery performance. I asked the friendly folks on WatchCrunch for a recommendation and the answer was GW-M5610.
I went to order the GW-M5610 (usually named GW-M5610-1 on sites) and found that it had been recently updated to GW-M5610U with an updated module and other features. And that is how I ended up with this gem.
The user guide calls the 4 buttons A-D, but I will be referring to them using their position on the watch since that is unambiguously easier:
Switching modes: Keep pressing BL repeatedly to switch between the following modes:
Changing settings: Be in timekeeping mode and press-and-hold TL for 2 seconds when current city appears in the city box. Pressing TL repeatedly will cycle through the following settings you can modify:
Solar charging: The display is surrounded by grey solar panels. The current charge is shown by the L/M/H (low/medium/high) bars at the bottom-right. The guide says that half an hour of direct sunlight can keep the watch running for a month.
Sync manually to atomic time calibration signal: Keep watch in upright position, preferably outside where it is easy to receive a radio signal. Press and hold BR until you see RCVD flashing in top-left corner. It also shows the last date-time when it had synced successfully while it is syncing. In the bottom-right it shows L1/L2/L3 depending how weak/medium/strong the signal is. If sync is successful, after about 10 minutes it shows GET in the top-right corner - press BR to go back to normal timekeeping mode. If sync has failed, it shows ERR in the top-right.