One fallacy about dating systems is the idea we should count years as 360 day intervals.  A "year" is a revolution of the earth around the sun, not 360 days.  Some people say this suggesting that the "Jewish calendar" has 30 day months, but there is no Jewish calendar that has 30 days months.  The fixed Jewish calendar alternates between 29 and 30 day months to keep in line with the lunar cycles (presently 29.5 days, not 30).
 
Personally, I think that since Genesis chapter 1 says the heavenly bodies are there to mark times and seasons, that the moon marks months and the sun marks years.  Thus if the Bible measures something in days or months we should go by lunar cycles (presently 29.5 days, not 30).  If it goes by "years", we should measure it by revolutions around the sun - that is, the earth returning to the same point around the sun.  This, not the calendar date, is what should reign supreme.
 
Now if a "prophetic year" is properly 360 days, then Passover, Shavuot, Yom Kippur, and other feasts would float through the year.  sometimes being in Spring, sometimes in summer ,etc.  So if we're going to calculate a 360 day year, then we should be calculating when Aviv is too.
 
God set things so the month of Aviv would always be in the spring, eliminating any "360 day" year usage when that does not agree with the solar cycles.
 
Until the time of Hezekiah the earth had a 30 day month and a 360 day year.  God changed that and how God might change it back since the end-time prophecies equate 1260 days, 42 months and 3.5 years as the same, like it was when the earth was first created.  Until the earth moves back to that rotation, it is silly to be looking for a 30 day month.
 
But independent of that issue, remember that the scriptures equate 1,260 days with 42 months and a "times, time and half a time" not a "year, years and half a year".  A "year", by a literal interpretation of Genesis chapter 1, would be a rotation of the earth around the sun (whether that be 360 days or 365+).  While a "time" might simply refer to the way a "year" is measured and dated, rather than an actual rotation.  Maybe that's why the Bible refers to it as a "time" instead of a "year".  There's not much argument that a "day" is considered from sunset to sunset, regardless of the length of the day, according to the definition of a "day" given in Genesis 1.  On any given day it might be a little less or a little more than 24 hours (though it will average out to that).  If sunset happens at 6:52 one day and 6:57 the next day, the "day" was 24 hours and 5 minutes long.  A "month" is however long it takes for the moon to go from new to new, whether that's 29 or 31 days.  So if the Bible talks about something measured in "years", and it's not further defined, I think we're talking about revolutions around the sun, no matter how many days may be involved in that.
 
So if 2000 years is really 1972 years because a "year" is really "360 days", then Pentecost / Shavuot could be any time of year and it would move about 5 days or so earlier in the year each year.  Thus during 2000 "prophetic eyars", Shavuot / Pentecost would move forward 400 days or 1 year and 40 days, making it about 30-50 days earlier in the year on this "prophetic calendar" that it would occur on a calendar where the months and years are kept in harmony with each other, putting Pentecost at about the time of Passover.
 
That of course, would be silly, since God made it clear Passover is always to start in the spring.  Thus, a "year" must revolve around the solar cycles, and not be compressed to some preconceived 360 day notion.
 
Shalom,
 
Joe