To switch from winter to summer time on the Casio G-Shock GA-2100-4AER, follow these steps: 1. Press the C button to enter the setting mode. 2. Use the C button to move the blinking display to the "CDST" setting. 3. Press the A button to select the "CDST" setting. 4. Use the C button to switch between the summer time options (on/off). 5.
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