If your PC wakes up after a power outage convinced it has traveled back in time, Windows may not be the only clock to investigate. A failing CMOS or real-time clock (RTC) battery is one possibility—but an incorrect time zone or a failed Windows time sync can produce a similar-looking problem.

The quickest way to narrow it down is to check the clock before Windows loads, then verify Windows’ own settings. This guide applies to Windows 10 and Windows 11; the firmware screens and battery-replacement procedure depend on your PC model.

An open desktop PC and date-and-time settings illustrate troubleshooting a motherboard’s CR2032 battery. First, check Windows time settings​

Connect the PC to the internet, then:

  1. Open Start > Settings > Time & language > Date & time.
  2. Turn on Set time automatically.
  3. Check the Time zone. If automatic selection chooses the wrong zone, turn Set time zone automatically off and select the correct one manually.
  4. Check the daylight-saving setting if your location observes it.
  5. Select Sync now if the button is available.

Microsoft documents these date, time and time-zone controls for both Windows 10 and Windows 11. If the date is very wrong and automatic synchronization fails, temporarily turn off automatic time, set the approximate date and time manually, then turn automatic time back on and retry the sync.

What success looks like: The displayed date, time and time zone match a reliable clock. But if the issue follows a power loss, don’t stop there: Windows may have corrected an incorrect hardware clock after startup.

Check the clock in UEFI or BIOS​

UEFI or BIOS is the firmware that starts before Windows. Checking its clock helps distinguish a Windows synchronization problem from a clock that loses its settings without power.

Save your work first. If the device uses BitLocker or device encryption, make sure you know how to retrieve its recovery key before troubleshooting firmware settings.

Enter firmware settings​

  • Windows 11: Open Settings > System > Recovery and select Restart now under Advanced startup.
  • Windows 10: Open Settings > Update & Security > Recovery and select Restart now under Advanced startup.

On the recovery screen, choose Troubleshoot > Advanced options > UEFI Firmware Settings > Restart. If that option is missing, consult your PC or motherboard manual for the startup key and instructions; the key varies by manufacturer. Microsoft notes that some recovery tools may require a BitLocker recovery key.

In firmware setup, find the system date and time. Menu names vary. If they are wrong, set them correctly and use the screen’s indicated option to save and exit. Avoid changing unrelated boot, security or storage settings.

Test whether the clock survives power loss​

For a desktop PC:

  1. Set the correct date and time in firmware, save the changes and shut down Windows.
  2. Unplug the PC from AC power for a short period—try about 10 minutes.
  3. Reconnect power and enter firmware setup before allowing Windows to start.
  4. Compare the firmware date and time with a reliable clock, allowing for the time that passed.

If the clock has reset to an old date or stopped advancing, the RTC backup battery or its supporting circuitry is a strong suspect. Dell identifies incorrect BIOS time and loss of firmware settings when powered off as common symptoms of a failing CMOS battery. This test points toward a hardware problem; it does not prove the battery alone is defective.

For a laptop, unplugging the charger alone may not remove all power because the main battery can still be connected. Follow the manufacturer’s model-specific guidance rather than opening the laptop or disconnecting an internal battery just to perform this test.

If the firmware clock stays correct but Windows is wrong: Focus on Windows time synchronization, time-zone settings and, on managed devices, your organization’s time configuration.

If Windows time synchronization fails​

First, confirm the PC has a working network connection and retry Sync now. On a work or school device, connect to the organization’s network or VPN as instructed by IT. Don’t replace an organization’s time server with a public one without approval.

For a closer look, an administrator can open Command Prompt as administrator and run:

Code: 
w32tm /query /status
w32tm /resync

The first command reports Windows Time status; the second requests synchronization. Microsoft identifies w32tm as its command-line tool for diagnosing Windows Time and notes that network restrictions, DNS problems or an unavailable time source can prevent synchronization. An error is a reason to investigate the connection or contact IT—not proof that the CMOS battery needs replacing.

Check or replace the CMOS battery​

If the firmware clock repeatedly loses time after the desktop is unplugged, consult the exact PC or motherboard service manual. It should identify whether there is a replaceable RTC battery, its type and how to reach it.

Many desktops use a coin-cell battery, but don’t buy one solely because it looks familiar. Dell, for example, specifies a CR2032 for its desktop guidance while also directing owners to model-specific documentation. Laptops and compact PCs can have different arrangements or require substantial disassembly.

If the manual permits replacement and you are comfortable opening the desktop:

  1. Record any custom firmware settings and make sure the BitLocker recovery key is accessible. Removing the battery can reset firmware settings.
  2. Shut down, disconnect all power sources and follow the manual’s safety and static-electricity precautions.
  3. Open the case as directed. Note the installed battery’s type and orientation.
  4. Remove it using the manufacturer’s method; avoid forcing the holder or prying with metal.
  5. Install a new battery of the specified type in the specified orientation, then reassemble the PC before reconnecting power.
  6. Enter firmware setup, restore the correct date and time and any necessary documented settings, then save and exit.

Dell advises replacing the battery with the same specified type and warns that removing it can erase BIOS settings. Dispose of the old battery according to local guidance, and keep coin cells away from children and pets.

Choose professional service instead if the battery is inaccessible, wired in, not clearly user-replaceable, or the device is managed by an organization. Seek service as well if a new, correctly installed battery does not fix the firmware clock: the holder, RTC circuitry or motherboard may need diagnosis.

Confirm the fix​

Repeat the unplugged desktop test and check the firmware clock before Windows loads. Then start Windows, verify the time zone and select Sync now again. A successful repair means the firmware clock retains the approximate correct time through the power interruption and Windows displays the correct local time afterward.

Windows 10 users can still follow these steps, but Microsoft’s standard support for Windows 10 ended on October 14, 2025.

 

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