I appreciate the comment, its probably one of the only positive ones I've received about my view which I do believe is Gods word.
The heptad is not approximate is spot on in that, the day for a year principle uses a conversion factor to go OT to NT, because we are looking at it from a 365.24 day year not a 360 day year. I believe the 1290years spanned from the time the daily sacrifice ceased during Daniels time (Daniel 12) under nebuchadnezzar in 23rd year of his reign in 583BC. The conversion factor is 360 day/year / 365.24 day/year = .9857, so you multiply 1290x.9857=1271.553. 1271.553-583BC=688.53. 688 is exactly to the dome of Rock.
1335 years I believe is even more spot on. Using the same day for a year conversion factor. 1335x.9857=1315.9095. 1315.9095-583BC=732AD. 732AD was the battle of tours which stopped the spread of Islam from overflowing into France etc.
Clemenslee: we have both concluded that the 1290 is the measure in years from the Babylonian cessation of offerings to the construction of the Dome. I suspect we arrived at this conclusion largely by the same path.
I had already done all the calculations you suggest several years ago; I discarded them in favor of 597/692 for some very sound reasons:
1. The 583 date for the final fall of Jerusalem has been totally discredited. The best scholars (and these are harmonizers [Thiele,etc] sympathetic to scripture) have settled on either 587 or 586 for Jerusalem's second sack. Israel's king chronology has now been fully (and rationally) harmonized within a couple months of actual history, and there is no longer any respected scholar who holds to 583.
2. The date found engraved on the inside of the Dome is 72 AH. This translates into 691/692 AD, not 688. Furthermore, recent scholarship has shown that most early Islamic monumental architecture records the command-to-build date, not the start of work or completion. Thus, there is a very good chance that 692 was the year Abd' al Malik gave the command for the Dome to be built. It is not certain, and it is very possible we shall never know for sure.
3. Using 597/692 allows us to ditch the "prophetic" year, and its decimal math. This is a good thing! It vastly simplifies all calculations. Now a 10 year old can do the math in a minute.
4. I know about Tours and Charles Martel. It does not qualify for the blessedness the angel assigns the 1335 in Daniel 12. There were any number of battles critical to saving Europe from the Muslim horde. The Battle of Masts was as big. So was Vienna. The angel is not speaking of temporary relief, but rather the permanent lifting of the Islamic threat, and the end of the Mosaic curse (the trampling of the land). Thus, this 1335 is not added to Nebuchanezzars sack, but rather to the AoD's date. The difference in length between Daniel's latter 1/2 heptad (the 1335) and the duration of John's first Beast, is the duration of John's second Beast (the 7th head). Play with it a bit, and you will see that John is providing an insanely accurate picture of Islam's history.
Thus, we are in basic agreement, but I think the dates I posit satisfy the prophecy, are more accurate to history, and are simpler to calculate.