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Fix incorrect time in Windows + OSX dual boot

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I just thought I'd give people an easy to find guide for this information. Hope the mods are alright with that, and that this doesn't violate any posting guidelines.

1. Why does this happen?


The simple explanation is; Windows uses Greenwhich time, and OSX uses Universal Time. It's as simple as that. Every time the other OS boots up - it changes around your BIOS settings in-order to 'correct' the CPU clock timer - which is essential for any computer to calculate things correctly.

2. What's the difference?

GMT


Windows is always defaulted to use GMT as the location to set the time and date. GMT stands for 'Greenwich Observatory'. This observatory broadcasts the current time of the world based on the calculations of where the Earth rotation is in relation to the prime meridian.

Because of the fact that GMT is outdated - it doesn't consider other factors that are important to calculating time - such as; where the earth is in relation to the sun. Because of this - GMT is pretty much an 'average' or an estimated guess as to what time it is where you live.

To quote the Greenwich website...


UTC

As more sophisticated time pieces became available to scientists, the need for a new international time standard became apparent. Atomic clocks did not need to keep time based on average solar time at a particular location because they were very, very accurate. In addition, it became understood that due to the irregularity of the earth and the sun's movements, the exact time needed to be modified occasionally through the use of leap seconds. With this precise accuracy of time, UTC was born.


UTC, while based on zero degrees longitude, which passes through the Greenwich Observatory, is based on atomic time and includes leap seconds as they are added to our clock every so often. UTC was used beginning in the mid-twentieth century but became the official standard of world time on January 1, 1972.


UTC is 24-hour time, which begins at 0:00 at midnight. 12:00 is noon, 13:00 is 1 p.m., 14:00 is 2 p.m. and so on until 23:59, which is 11:59 p.m.


Time zones today are a certain number of hours or hours and minutes behind or ahead of UTC. UTC is also known as Zulu time in the airforce. When European Summer Time is not in effect, UTC matches the time zone of the United Kingdom.


Today, it is most appropriate to use and refer to time based on UTC and not on GMT.

3. How do I fix this?

It's quite simple really. All you need to do is set Windows to use UTC instead of GMT. To do this - we need to perform a simple registry edit. Go perform this - hold down the 'Windows' button, and at the same time press R (Win + R) this will bring up a new window titled 'RUN'

In this window type the command 'regedit'. UAC will popup asking for admin permission. Click accept and the registry editor will open. Now that were in regedit, navigate to...



Left click on 'TimeZoneInformation' and on the left it will display all the current values attributed to this registry key. On the left right click anywhere and choose 'Create new D-WORD'. You will now see a new entry at the bottom of the list.

Right click on this new entry and choose 'Modify'. Rename the key 'RealTimeIsUniversal' and change it's value to '1'. Click save, and now the entry should say 'RealTimeIsUniversal - 0x0000001' If it looks like that - then you're all done!

Reboot, and sync the windows clock - now your time settings will be universal (no pun intended :p ).

EDIT:

If you don't like fooling around with your registry, I've created a .reg file that will do all the work for you. All you need to do is download the attachment, extract it to where ever you want, and double click on 'Windows Universal time On'

If you want to switch back to GMT. Do the same, but instead - double click on the reg file named 'Windows Universal time Off' and that will remove the registry edits we made. :thumbup:

Anyways, hope this guide helps someone! Thanks for reading everyone!! It's greatly appreciated! I'm definitely looking forward to a good tenure on this very helpful forum. :)

View attachment 94625

I just thought I'd give people an easy to find guide for this information. Hope the mods are alright with that, and that this doesn't violate any posting guidelines.

1. Why does this happen?


The simple explanation is; Windows uses Greenwhich time, and OSX uses Universal Time. It's as simple as that. Every time the other OS boots up - it changes around your BIOS settings in-order to 'correct' the CPU clock timer - which is essential for any computer to calculate things correctly.

2. What's the difference?

GMT


Windows is always defaulted to use GMT as the location to set the time and date. GMT stands for 'Greenwich Observatory'. This observatory broadcasts the current time of the world based on the calculations of where the Earth rotation is in relation to the prime meridian.

Because of the fact that GMT is outdated - it doesn't consider other factors that are important to calculating time - such as; where the earth is in relation to the sun. Because of this - GMT is pretty much an 'average' or an estimated guess as to what time it is where you live.

To quote the Greenwich website...


UTC

As more sophisticated time pieces became available to scientists, the need for a new international time standard became apparent. Atomic clocks did not need to keep time based on average solar time at a particular location because they were very, very accurate. In addition, it became understood that due to the irregularity of the earth and the sun's movements, the exact time needed to be modified occasionally through the use of leap seconds. With this precise accuracy of time, UTC was born.


UTC, while based on zero degrees longitude, which passes through the Greenwich Observatory, is based on atomic time and includes leap seconds as they are added to our clock every so often. UTC was used beginning in the mid-twentieth century but became the official standard of world time on January 1, 1972.


UTC is 24-hour time, which begins at 0:00 at midnight. 12:00 is noon, 13:00 is 1 p.m., 14:00 is 2 p.m. and so on until 23:59, which is 11:59 p.m.


Time zones today are a certain number of hours or hours and minutes behind or ahead of UTC. UTC is also known as Zulu time in the airforce. When European Summer Time is not in effect, UTC matches the time zone of the United Kingdom.


Today, it is most appropriate to use and refer to time based on UTC and not on GMT.

3. How do I fix this?

It's quite simple really. All you need to do is set Windows to use UTC instead of GMT. To do this - we need to perform a simple registry edit. Go perform this - hold down the 'Windows' button, and at the same time press R (Win + R) this will bring up a new window titled 'RUN'

In this window type the command 'regedit'. UAC will popup asking for admin permission. Click accept and the registry editor will open. Now that were in regedit, navigate to...



Left click on 'TimeZoneInformation' and on the left it will display all the current values attributed to this registry key. On the left right click anywhere and choose 'Create new D-WORD'. You will now see a new entry at the bottom of the list.

Right click on this new entry and choose 'Modify'. Rename the key 'RealTimeIsUniversal' and change it's value to '1'. Click save, and now the entry should say 'RealTimeIsUniversal - 0x0000001' If it looks like that - then you're all done!

Reboot, and sync the windows clock - now your time settings will be universal (no pun intended :p ).

EDIT:

If you don't like fooling around with your registry, I've created a .reg file that will do all the work for you. All you need to do is download the attachment, extract it to where ever you want, and double click on 'Windows Universal time On'

If you want to switch back to GMT. Do the same, but instead - double click on the reg file named 'Windows Universal time Off' and that will remove the registry edits we made. :thumbup:

Anyways, hope this guide helps someone! Thanks for reading everyone!! It's greatly appreciated! I'm definitely looking forward to a good tenure on this very helpful forum. :)

View attachment 94625

Thank you so much, this has always been an irritating problem. Used the attachment, worked perfectly.
 
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My many years problem is gone. My big thanks to You. :)
 
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Thanks you helped me. It was no big thing for me but on the other hand it was really annoying:thumbup::thumbup:
 
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That explains it, I figured it had something to do with that! thanks for the fix!!
 
Joined
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Lisa
There was a time-toggle.pkg for Mac to tweak BIOS time to local timezone, but the coding was bad, just shifting the timezone difference upon each boot. Changing Windows registry is the way to go.
 
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Thanks for that info SwampFox82.
 
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Anyone knows how to solve it when 3rd boot option is a linux?
 
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