The ACPI Time and Alarm (TAD) device is an alternative to the Real
Time Clock (RTC). Its wake timers allow the system to transition from
the S3 (or optionally S4/S5) state to S0 state after a time period
elapses. In comparison with the RTC Alarm, the TAD provides a larger
scale of flexibility in the wake timers. The time capabilities of the
TAD maintain the time of day information across platform power
transitions, and keep track of time even when the platform is turned
off.