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I'm attempting to work through this issue where a Windows 8.1 PC regularly loses track of time, sometimes going from (as an example) 1pm on one day to 8am on a previous day. The values vary.

Every time I check the BIOS clock when it's wrong in Windows, it's the same value in the BIOS. Super wrong. Every time I manually sync to the time server in windows, the BIOS will reflect the time change.

What is the relationship between the real-time clock and the OS-level clock? Does Windows sync only serve to correct the clock, but then run off of what the mobo is giving it afterwards? How could something occur like this?

As a note, this happens while the computer is on and fully functioning, so I don't think whether or not the CMOS battery is good matters, because this has nothing to do with booting up with the wrong time. This happens while the computer is running.

(I saw a related question on How time is measured in computer systems but it didn't seem to answer this question)

PC is a custom-built, with a Z87 Pro Asus Mobo.

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    The CMOS battery could still be the culprit; try unplugging all power from the PC and see if it still maintains the last time value (or BIOS settings). The CMOS battery is there in case the system loses all power, but some use it still specifically for the RTC; you don't make mention of your MoBo brand/type so it's hard to say how they designed it .. still worth a look since that's usually the easiest/cheapest fix .. next thing to check would be the power supply, it could have a bad rail that's causing the RTC to not get the proper voltage and lose sync ..? Is it a laptop? – txtechhelp Jul 10 '15 at 5:12
  • @txtechhelp interesting, I was not aware a bad rail could be affecting time like this. I updated the question with mobo information, thanks for reminding me I didn't do it. – GarrettJ Jul 10 '15 at 5:26
  • No worries; that's a nice motherboard by the way, very jealous :) .. Are you overclocking anything? That shouldn't mess with it, but that could be one possibility .. A Windows 8 driver (or other possible software) could also be the culprit if the CMOS battery isn't .. and a bad power rail "could" be another possibility since the clock needs a constant voltage to keep "accurate" time .. clock sync issues like this are usually (luckily) only the cause of a few things that are easy enough to track down .. – txtechhelp Jul 10 '15 at 6:32
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There are to options: Set BIOS time to localtime or to UTC. Everyone uses the latter, only Windows uses localtime. So in Windows it's just that Windows gets the time from a NTP server, calculates localtime and writes it to the BIOS. On UNIX/Linux/etc. the UTC is used for the BIOS time and the OS just adds a timeadjustment offset to calculate localtime.

How to "fix" Windows into using UTC via registry is described here: Archwiki: Time#UTC_in_Windows

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