Rafael J. Wysocki of Intel submitted a patch to Linux ACPI for a ACPI Time and Alarm Device (TAD) driver:
Introduce a driver for the ACPI Time and Alarm Device (TAD) based on Section 9.18 of ACPI 6.2. This driver only supports the system wakeup capabilities of the TAD which are mandatory. Support for the RTC capabilities of the TAD will be added to it in the future. This driver is entirely sysfs-based. It provides attributes (under the TAD platform device) to allow user space to manage the AC and DC wakeup timers of the TAD: set and read their values, set and check their expire timer wake policies, check and clear their status and check the capabilities of the TAD reported by AML. The DC timer attributes are only present if the TAD supports a separate DC alarm timer. The wakeup events handling and power management of the TAD is expected to be taken care of by the ACPI PM domain attached to its platform device.
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.
More info: linux-acpi list archives, http://vger.kernel.org/majordomo-info.html