The nitty gritty of the rationale for 607 is hairsplitting and logical gymnastics of the highest order with a good dose of suspension of disbelief. It is a classic case of the FDS quoting historians when it suits their argument and ignoring them when there is simply no evidence to support their view. When reality-checks threaten to disrupt the continuum of fantasy (I mean really--7 times multiplied by 360 days per lunar year with a day for a year each demands three separate unrelated interpretive mechanisms!!) the mind of the JW has a choice, to reel or retreat. The first forces questioning, deeper investigation, the seed of doubt, and the only there thing to combat it is the usual mind-control devices--the FDS knows what's best as the Voice of Jehovah; your doubts and questioning are Satanic; pray for further insight because you don't get it; wait on Jah or FDS; talk to the elders; etc etc etc. (http://www.607v587.com/ is an example of one JW whose mind reeled and pursued it vigorously, with disappointing but predictable results.) Retreat into the comfortable bosom of congregational good graces, warmth and acceptance while these questions go unanswered can seem easier, but there's a psychic price to pay. So many JWs today (a lost generation indeed) still cling desparately onto the significance of 1914, for so long as that date remains pivotal then given the method of arrival at that date 607 won't change.
Oddly, I once read that it was proposed by a member of the GB (was it in Crisis of Conscience?) that 1957 should become the new date for the beginning of the "last days" because of Sputnik!! So look for further tweaking of this doctrine sometime soon.