Apple II system clocks, also known as real-time clocks, were commodities in the early days of computing. A clock/calendar did not become standard in the Apple II line of computers until 1986 with the introduction of the Apple IIGS. Although many productivity programs as well as the ProDOS operating system implemented time/date functions, users would have to manually enter this information every time they turned the computer on. Power users often had their Apple II's peripheral slots completely filled with expansion cards, so third party vendors came up with some unique solutions in order to mitigate this problem with products like the Serial Pro and No-Slot Clock.