6 hours function

I'm working on a system and i want the arduino to execute a function each 6 hours i want to know the most reliable way to do that without wasting the arduino MC time .Ithink about
1- timers but i found the maximum time for a timer is 8S in this library Arduino Playground - HomePage
2-using Millis() or Micros() but i will waste my arduino time in polling for millis or micros values.
3-time library but i don't know if it will be suitable as the arduino will work without using serial port or any other peripherals.
could any one help me please.
Many Thanks in advance.

i will waste my arduino time in polling.

What else where you planning on using the wasted time for?

1- timers but i found the maximum time for a timer is 8S in this library

Did you mean the watchdog timer?
Eight seconds is only an approximate time, but there are around 2700 such timeouts in six hours. You could count them.

Well 6 hours is a long time in clock cycles, but it may be possible with a realy slow clock and really big counter. Overflow causes and interrupt in the arduino, then you do what you need to do.

Maybe an RTC with alarm that wakes up (with an interrupt) your sleeping Arduino every 6 hours?