That model number you quoted of JRSI223 isn't coming up.
This page will tell you where to find the model and serial number on your Jenn-Air fridge:
Locate Model Number & Serial Number
Just click on the proper link depending on whether it's a side-by-side fridge or not.
Then, use this link to find the nearest factory authorized Jenn-Air repair depot to you:
Service & Repair
They should be able to tell you where the defrost timer is. Also, ask them how to gain access to the evaporator so that you can check the defrost heater element.
On the fridges I'm familiar with, the defrost timer is located behind a small removable panel in the roof or side of the fresh food compartment. But, they could have hidden it anywhere, including behind the fridge near the compressor or behind the front toe kick.
You might not need to replace the defrost timer. Frost forming in your freezer compartment is definitely an indication that your defrost cycle isn't working, but that doesn't necessarily mean that the defrost timer isn't working properly. It could be that your defrost heater is gone.
If this is a regular frost free fridge and not a side-by-side, you should see a panel somewhere in your freezer compartment. Removing that panel will allow you to access the evaporator coil. Near that evaporator coil will be a heating element. That will be the heating element that the defrost timer turns on to defrost the evaporator. See if it's still in one piece, and with the fridge turned off, check for continuity across it with an ohmmeter. If the defrost heater needs to be replaced, then it's considered "good practice" in the industry to replace the defrost thermostat at the same time as the defrost heater. The reason why is that the defrost thermostat might cost $5 or so, but it protects a lot of styrofoam parts in the area of the evaporator that are both expensive and require an awful lot of work to replace. Consequently, appliance repairmen will replace the defrost thermostat when they replace the defrost heater just as a preventive maintenance measure.
If the defrost heater looks fine, then the problem most likely is that the defrost timer is sticking in the "run" mode, and isn't defrosting the fridge.