If this is completely new to you, I'll try and walk you through the steps.
What you do is find your mission folder.
If it happens to be an editor mission, that folder is in your documents.
(I believe like this "C:\Documents and Settings\Your Name\Arma\Missions", I am not entirely sure.)
In that folder create a text document.
Name the document so that it is "name_of_script.txt",
naming it whatever you want to.
Now, change the ".txt" part to ".sqf".
Open the text document and enter the following
// (// at the beginning of a line marks that line as a comment)
// Script for maintaining time
_today = date;
while {true} do
{
setdate _today;
sleep 10;
};
In your mission (in the actual editor) put in the players init line:
[] execvm "name_of_script.sqf".
All it does is find the date and time, saves it as a variable (today),
and as long as the condition for while is satisfied, (in this case true, which will continue the loop infinitely)
the coding resets the time to the instant the script started, resetting back after every 10 seconds.
(The sleep 10 controls the interval in seconds, you can change it,
but if you delete it, your computer will begin to lag severely.)
Hope that all helps.
Luke