This is not a precise calculation though very interested.

Suppose as we knew and Mike pointed out that only the second 3.5 years will be shortened means we still have 42 months and 30 days each month but only 16 hours a day.
Then 42 months in new calender = 28 months in old calender.
While assuming that the first 3.5 years still like now.
Then we will have:
42 + 28 = 70 months only instead of 84 months!!!
See the sacral 70 number in it!!!
70 months equal 5 years 10 months.
Deducting this from 2017 we will have February 2012 or around as the start of the Great Tribulation. But this assume 2017 January as the coming of Messiah suppose it is on September/October 207 then we have possible range of February to November/December 2012 as the beginning of tribulation.
Interesting, it coincides with Mayan December 2012 and it is in 2012 too ....
Just wait and see about it but I feel strongly that this is kind of the right calculation ....

This is of course using assumption of 7 years tribulation period which then valid under this approach.

In short because we have only 16 hours a day in the last 3.5 years great tribulation then
42 months in new calender = 28 months in old calender (our current calender).
We will then only have 42 + 28 = 70 months only (in our current calender as only to indicate the length of time not the time itself) instead of 84 months!!!
See the sacral 70 number in it!!!
70 months equal 5 years 10 months.
So if the rapture is on September/October 2017 then we have possible range of February to November/December 2012 as the beginning of tribulation.
.. Maybe this is really the start of tribulation .... which then put even Ron Reese rapture in 2012 still in the right time frame ....though different approach ...

It could be possible Gimel with your calculation provided 6th seal is after 42months and the shortening period happens as what you posted.

If using a possible timeframe that this year could literally begin 6th seal event...then the 42months are really what you mentioned like 28months[current calculation]

With this and since BHO was already 37months in office...could it be that 7months would be needed for the Temple to be dedicated or dragon to cast down or the day BHO sit and start the abomination of desolation in the 7month and stopped sacrifices?
37+7is 42 and so BHO continues 42months more...

Revelation:13:5: "And there was given unto him a mouth speaking great things and blasphemies; and power was given unto him to continue forty and two months."

Revelation:17:10: "And there are seven kings: five are fallen, and one is, and the other is not yet come; and when he cometh, he must continue a short space."

Mike are you saying 2012 November election or this coming passover 2012?
Many thought that 2011 should be something, and I am still thinking so even not the rapture itself but something so important should happen in 2011 and the signal for 2012 and it must be a very strong signal. One of them is the Euro financial crisis looming in the horizon, the coming world economic depression. The other signal would be related to natural disasters other than Japan disasters which is not happened yet, something really weird should happen but I don't know it.

Anyway, your calculation might be true and hidden from many ....

Obama I think has a strong chance to have another 42 months, that is why Sarah Palin doesn't want to contest with him, for she might know who is O actually by the Holy Spirit ...
Martial law I think is also on the horizon not because of the internal but because of Iran's war which will come in just weeks even days from now.