Shoutbox

You could do this by getting the JScript to get the time and add the specified amount of time.  You can then put this value in the registry with the name of your contact and get a timer to keep checking this to see if it is past this time.  This script would take a bit but is possible.  One limitation though is it can only be a precise as the timer you set ad the shorter the timer, the more resources you use of your computer.

EDIT: looks like i was beaten to it