Nikon D5100 Reference Manual page 38

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The Setup Menu
Language and date/time settings can be changed at any time using the Language (0 171)
and Time zone and date (0 170) options in the setup menu.
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The Clock Battery
The camera clock is powered by an independent, rechargeable power source, which is
charged as necessary when the main battery is installed or the camera is powered by an
optional EP-5A power connector and EH-5b AC adapter (0 208). Three days of charging will
power the clock for about a month. If a message warning that the clock is not set is displayed
when the camera is turned on, the clock battery is exhausted and the clock has been reset.
Set the clock to the correct time and date.
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The Camera Clock
The camera clock is less accurate than most watches and household clocks. Check the clock
regularly against more accurate time pieces and reset as necessary.
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