Keeping track of the current time and date is very crucial, not just to let the user know about it but to use it as a timestamp for various resources in the system, specifically, files present in secondary storage. Every file has metadata information such as the date of creation and last modification date, and every time a file is created or modified, these two fields are updated with the current time in the system. These fields are used by several apps to manage files such as to sort, group, or even delete them (if the file hasn't been accessed a for long time). The make tool uses this timestamp to determine whether a source file has been edited since the last time it accessed it; only then is it compiled, otherwise left untouched.

The system clock RTC keeps track of the current time and date; backed by an additional battery, it continues to tick even when the system is turned off.

RTC can raise interrupts on IRQ8 periodically. This feature can be used as an alarm facility, by programming the RTC to raise interrupt on IRQ8 when it reaches a specific time. In IBM-compatible PCs, the RTC is mapped to the 0x70 and 0x71 I/O ports. It can be accessed through the /dev/rtc device file.