I read the Post article. It says that GW recieved an early discharge eight months before his six year committment was up. The missing time people are saying that he went AWOL 17 to 18 months before his six years were up. So there still seems to be about 10 months of missing time, even taking the early discharge into account. Here's the relevant quote from the Post article:

To start at Harvard, Bush needed early release from Guard duty in Texas, and he got it easily, about eight months short of a full six years. A Bush spokesman, Dan Bartlett, said early departures were quite common and, in Bush's case, appropriate because his unit had phased out the F-102s. Bush was transferred to a reserve unit in Boston for the rest of his time, Bartlett noted.