, so can you tell me how much reliable is my Arduino Uno?
It's very reliable... just as reliable as any other electronic gadget... as long as your code is bug-free...
I dont want it to turn off or so... Can I use delay function?
Yes, but since "nothing happens" during the delay time you won't know if it's working unless you wait 12 hours. You might want to blink and LED or something. In that case use millis()
DS1302 is very unaccurate)..[/quite]No clock is perfect, but I assume the DS1302 is more accurate than the Arduino's clock.
Or can I use millis function? How to use it(I'm new to programming)? If you have any ideas, please write code down and explain it to me(I want to learn it :))..
Just modify the [u]Blink Without Delay Example[/u] for a much-much longer delay. Please double-check my math, but I get 12 hours = 43,200 seconds = 43,200,000 milliseconds.
If you want to do blink an LED at the same time, you'll need two timers (two different interval variables and two different previous variables). Look at the post at the top of the forum that says "Demonstration code for several things at the same time."