Citizen E87 Manual page 5

Before using this watch
After unpacking, the following two must be carried out before using the watch:
◆ Checking the power reserve → Page20
For optimal performance, ensure your watch is fully charged prior to use.
◆ Setting the time and calendar → Page24, 26
Set the correct time and calendar.
4
This watch has a rechargeable cell which is charged by
exposing the dial to light.
Expose the dial to light regularly to charge the watch.
For details of charging, see pages 18 to 23.