When the Windows clock is wrong - the time at the right hand side of the taskbar - there is always a reason. In my case, I noticed that the clock was an hour slow one day. I set it correctly and later that day I discovered that it had reverted back to being one hour behind again.
There are several possible causes of a problem like this and the first thing to do is to synchronise the PC's clock with a time server on the internet. This is basically a clock on the internet that replies with the correct time when it is accessed. Windows uses one to set the PC's clock and it should happen automatically, but sometimes it can go wrong.
Click the clock at the right hand side of the taskbar and then click Change date and time settings. You could click the Change date and time button on the Date and Time tab, but there is an easier way. select the Internet Time tab and click Change settings.
A new window opens and there is a drop-down list of time servers. the default time.windows.com should work fine, but if there is a problem for some reason, you could select one of the others. Click the Update now button and the Windows clock will be set.
In my case it did indeed set the time correctly, but a few hours later the clock was found to be an hour slow.
The next step is to check the regional settings and the computer could think that it is in a different time zone, in my case, one that is one hour behind local time. Left click (Windows 7) or right click (Windows 8) the Start button and select Control Panel on the menu. Switch to Small icons view and click Region. Make sure that the Location tab is set to your location. In my case, the region was correct and, when synced, the time was correct.
A battery inside the computer keeps the clock ticking when the computer is switched off. When this fails, the time is either random, or a default value like 1st Jan, 1997 or something similar. It would not be exactly one hour slow as was the case with my computer. Replacing the battery on the motherboard is easy with some computers, but very difficult with many, especially laptops, which are hard to disassemble.
The cause of the clock running an hour behind, as I realised eventually, was Linux. I had been running the latest version of Ubuntu Linux, but not an installed copy, instead it was a live CD. (Actually I was running Ubuntu from a USB flash drive.)
When you install Linux, it always asks you to set your time zone, but the Ubuntu Live version does not install and it boots up straight to the desktop from the DVD or USB thumb drive. This does not set the region and the time was taken from the computer, so initially it was correct.
When I connected to the Wi-Fi network and established an internet connection, it went online, got the time, and set the PC's clock. The time it downloaded was the wrong time zone and was one hour behind my local time. I didn't notice until I rebooted back into Windows and looked at the clock in the taskbar.
I also discovered that even if you run Linux inside a virtual machine such as VirtualBox, it can set the PC's clock. It must have access to the system time. You must set it correctly by going to the Time & Date settings. Just mouse over the time in the top right corner of the screen.
The lesson to be learnt here is that there is always a reason why the PC's clock is wrong and you just have to find it. The cause might be something unusual, such as running a Linux distro as in my case.