The emissions timer gets a signal from the odometer. At the specified number of miles (I remember it being 82,500, but I may be a bit off), it turns on the light on the dash.
The purpose of this was to insure that when your typical XJ reached the 80K mark, the owner would be motivated to take it in for replacement of the Oxygen Sensor, which dies about that mileage.
The "official" way to get the light turned off is to use the Chryco diagnostic module that plugs into the ECU circuit. All it does is disable the timer so the light will go out.
My recommendation, unless you want to pay the dealer to defeat the lamp with the diagnostic tool, is to unplug the timer, and if you are at all concerned, put in a new O2 sensor.