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System time is a representation of the way that computers measure the passage of time. The time and calendar date displayed by an operating system (OS), or arrived at by various programming languages, is generated using system time. This is performed by a system clock that counts out ticks since an arbitrary starting point known as an epoch. Each operating system uses its own epoch and ticks that represent different lengths of time. By determining how many ticks have elapsed since the epoch, and then converting them into seconds, it is possible for a computer to render the current time and date.
Early personal computers (PCs) did not have any way to keep track of time. The first models to have that functionality were manufactured by International Business Machines (IBM). Since then, all PCs and OSes have included some way to keep track of time and convert it into a form recognizable to humans. With the advent of networking, and especially the Internet, PCs also became capable of adjusting system time to account for time zones. It also became possible to update the system time to match a central server.
The way that computers typically measure time is by counting ticks since an arbitrary starting point. Ticks are not universally convertible to a measurement of time useful to humans, since each operating system allows a different amount of time to pass between them. Some systems count one tick for every 100 nanoseconds, while others equate one tick to one second. If a system operates concurrently for a long enough time, it is possible for the tick count to reach the maximum number of digits allowed and wrap around to zero. In some cases this may cause system instability.
Each system also has a different starting date that ticks are counted from. Some operating systems use an epoch that begins in 1601CE and has a range that lasts until 2099CE. Others, such as system basic input/output (BIOS) use an epoch that counts from midnight on the current day. In each case, it is possible for a computer to count how many ticks have occurred since the epoch began and then convert that figure into a calendar date and time that is useful to the human user. The resolution that system time is capable of returning is dependent on the length of a tick, so some systems can report in milliseconds while others are limited to the nearest second.