I'll show you how to flip time zones on your Casio G-Shock, so your World Time city becomes your local time and vice versa. If you're traveling to a new city, this is the best and Step 1: Press and hold the top left button of Casio G-Shock wrist watch while in regular Timekeeping Mode. Step 2: Wait for the City Code on the display to start flashing, indicating that you have entered the correct setting screen. Step 3: Use the bottom right button on the Casio G-Shock to highlight the desired City Code for your time zone. Current models of G-shock change the time zone automatically, so the first thing you are prompted to do is choose a city code for where you live or at least for a city in the same time zone. Setting the time zone on your Casio G-Shock wrist watch is a simple task that can be completed by choosing the correct City Code for your particular time zone. You can correct the time zone for your Casio G-Shock in less than five minutes. Press and hold the top left button your Casio G-Shock wrist watch while in regular Timekeeping Mode. 1 G-Shock Button Arrangements. 2 Changing The Time. Figuring out how to change time on a G-Shock watch is difficult and frustrating. One might not have the manual, or just can't be bothered reading it! G-watch zone has you covered step by step guide how to adjust the time on G-Shock GA-100. VrJpXVS.