You could try using:
prontera,153,167,6 script NPC 100,{
mes "Hi";
set .Get,1;
close;
OnHour00:
set .Get,0;
}
When your server hits the 00 Hours, it will set the variable to 0.
Now, i don't know if the NPC it's attached by default, but the most probably it's that it's attached by default.
(in other words, you don't need to worry, the script will work fine).
Good Luck!