Most of the people I've talked with assume that each year of TOS corresponds to a year of the Mission. A lot of fan productions appear to operate under this assumption as well. It is my contention that the three season of TOS comprise the entirety of the Five Year Mission. Many episodes take days or weeks to complete. Consider that The Paradise Syndrome takes over a month in it's self. How long where Kirk and Spock trapped on Organia? Seems like a week at least, if not longer. Is there any internal evidence that supports this one way or another?