System time is measured by counting ticks from an arbitrary starting point known as an epoch, with each operating system using its own epoch and ticks. PCs now have the ability to track time and adjust it based on time zones. The length of a tick determines the resolution of the system time.
System time is a representation of how computers measure the passage of time. The calendar date and time displayed by an operating system (OS), or obtained from various programming languages, is generated using the system time. This is done by a system clock which counts ticks from an arbitrary starting point known as an epoch. Each operating system uses its own epoch and ticks which represent different lengths of time. By determining how many ticks have elapsed since the epoch, and then converting them to seconds, it is possible for a computer to render the current time and date.
Early personal computers (PCs) had no way to track time. The first models to have this feature were produced by International Business Machines (IBM). Since then, all PCs and operating systems have included a way to track time and convert it into a form that humans can recognize. With the advent of the network, and especially the Internet, PCs have also become capable of adjusting system time based on time zones. It was also possible to update the system time to match a central server.
The way computers typically measure time is by counting ticks from an arbitrary starting point. Ticks are not universally convertible into a measure of time useful to humans, as each operating system allows a different amount of time to pass between them. Some systems count one tick every 100 nanoseconds, while others equate one tick to one second. If a system runs concurrently long enough, it is possible for the tick count to reach the maximum number of digits allowed and drift towards zero. In some cases this can cause system instability.
Each system also has a different start date from which ticks are counted. Some operating systems use an epoch that starts in 1601 AD and have a range that lasts up to 2099 AD Others, such as the basic input/output system (BIOS) use an epoch that counts from midnight of the current day. In any case, it is possible for a computer to count how many ticks have occurred since the beginning of the epoch and then convert that figure into a useful calendar date and time for the human user. The resolution that system time is able to return depends on the length of a tick, so some systems may report in milliseconds while others are limited to the nearest second.
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