Your refrigerator does not have a timer, but uses an electronic board to control defrost. If your evaporator is not defrosting, I am assuming you have found it covered in ice. Remove the lower panel that covers the evaporator and check the defrost thermostat for continuity. If it reads open, then that is the problem as it should be closed if it is stuck in a block of ice. You can also check the resistance of the element too, but the thermostat is the likely failure. Replace the part, then use hot water and a hair dryer to remove the ice from the freezer section.

The circuit board can fail like anything else, but it is rare. The board in your refer controls everything so that tends to decrease the odds that only one section has failed. Not saying it can't happen, but much less likely. The board can be checked by performing the test from the tech sheet and then check for voltage at the element or coming out of the board. If you have voltage, then you are back to the element or the thermostat.