What do you mean by new linkcvar?
The linkcvar command just uses whatever cvar is in the game in a hud resource. And you can create a new cvar with "set mynewcvar newcvar_value"
If you want to get your computer time in a .urc file, that is not possible. You will need an external program that injects and update a new cvar with your local time to your game.
And btw, that mod was already done by Elgan check it out MOH Clock HUD