NTP – Network Time Protocol
If you have ever noticed that the time on your computer tends to drift away from the actual time, do not be surprised. Computers keep their time internally, but like many watches and clocks, they are not always completely accurate. Instead, they are off just a little bit. Over time, this means that the clock may be off by several minutes, or hours.
To deal with this, all computer operating systems that I am aware of, have the ability to reset the time to the correct time. In most cases, this is done using the Network Time Protocol, or NTP. Different operating systems, implement this in different ways. However the concept is the same in all cases.
Usually, the best setup is to have one, or two, of your computers synchronize time one of the many publicly available time servers. These are time servers that synchronize to master time servers, and therefore can be relied on to have the correct time.
Once you have one, or two, of your local computers keeping the correct time, then they are the ones that the rest of your computers synchronize to. This reduces the load on the public time servers, as well as your internet connection. The network load for time synchronization is small, so the load on your Internet connection is not that big of a deal. Not overly impacting the public time servers is a courtesy, and a requirement by some of them to be allowed to use them.
With all of your computers having the correct time, such things as sales receipts, emails, and so forth, will always have the correct time stamp.