The break point is 31 July 2014. Any ATP written passed before that date has a 24 month shelf life. If you pass the written after that date you have a 60 month shelf life and there is no shelf life for air carrier employees.

Technically if you pass the written before 01 August 2014 but after completing an ATP CTP you get the 60 month window. But you don't get the air carrier extension unless you pass the test after the end of July.

One other thing to keep in mind. When you read 61.156 you'll notice it says "After July 31, 2014, a person who applies for the knowledge test for an airline transport pilot certificate with an airplane category multiengine class rating..." Currently the airplane ATP written does not differentiate between single and multi engine. It looks like the FAA is going to overhaul the ATP written. It may take the test prep folks a while to get spooled on the new tests.