Hi and welcome to FixYa. I am Kelly.
Actually it sounds like the thermostat and the timer are working just fine. What appears to be happening is the timer is starting the defrost cycle but the defrost heater is not working. This will result in a horribly long defrost time until the freezer compartment reaches 60 deg F and the defrost thermostat at that point terminates the defrost cycle.
Defrost element Item 9 this link:
Defrost thermostat item 12 above first link
You should read the resistance of the defrost heater by reading the yellow to blue wire (terminals 2 to 3) on the defrost control when the freezer comparment is below 60 deg F at the evaporator
. No resistance = open defrost heating element. This is reading continuity though both the defrost thermostat and the heating element. If you do not get any resistance and you are sure that the freezer is well below 60 deg F then... and only then would the defrost thermostat be suspect.
Defrost control in the overhead of the fresh food compartment is located aft of the lights on the thinner section cand an be manually cycled by rotating the 2 tabs on the face of the timer dial. It will click and stop the compressor and evaportor fan ONCE each turn.
NOTE you must wait 10 to 15 minutes after cycling this so that the compressor will stabilize and restart.
Once the comprssor and fan shut off the heater is supposed to turn on and melt the ice on the evaporator until the defrost thermostat reaches 60 deg F and once again provides power to the defrost timer motor to terminate the defrost cycle and restart the compressor.
Defrost timer is item 15 this link:
This should be enough information to get you going. I still think that your defrost heater is not working either from failure or a wiring problem.
Thanks for choosing FixYa,