The Wake up task prepare a computer for Wake up at a specific time or after a certain time.
At a certain date and time
Wakes up the computer at the specified year, month, day, hour, minute and second.
At a certain time
Wakes up the computer at the specified hour, minute and second.
After a certain time
Wakes up the computer after a certain time; hour, minute and second.
Remark on Wake Up: Some computers can wake up from standby and not from hibernation, some work for both, and some for none. It all depends on the BIOS and the APM/ACPI capabilities, your motherboard, and your ATX power supply version.