The term "prophetic year" is a construct from later biblical interpretation, not found in the Bible itself. What the Society is generally unaware of is that ancient Judea had two different calendars, both well-attested: (1) The 354-day lunisolar calendar, the one that survives today as the traditional Hebrew calendar and (2) The 364-day schematic priestly calendar, which was characteristic of Enochic and Essene Judaism, but which may have been used earlier in Zadokite Judaism (such as in P and Ezekiel; cf. P's chronology of the Flood). Because the schematic calendar contained 364 days, it was evenly divisible by 7, and thus was sabbatical and avoided conflicts with the sabbath. Four of these days were epagomenal, i.e. the two solstices and the two equinoxes. There were two ways of reckoning this calendar. The older method, attested in the Book of Luminaries of 1 Enoch (third century BC), placed these four days outside the months as special markers of the seasons; each season would consist of three 30-day months, followed by an epagomenal day. The later method, attested in Jubilees and at Qumran, simply lumped this day with the final month of the season; thus each season would consist of two 30-day months and one 31-day month. The earlier method thus reckoned only 360 days in the months but the year was completed in 364 days. The latter method reckoned a full 364 days in the months. The question with Daniel (early second century BC) is, which calendar does it reflect and which system of reckoning? It is the schematic calendar without epagomenal inclusion that reckons the months as only 30 days in length (compare Daniel 6:7, 12 which has a 30-day period as a set duration), thus Revelation 11:2-3 correctly states that 1,260 days equal 42 months. The durations of 1,290 (+ 30) and 1,335 days (+ 30 days + 30 days + 15 days) also confirm this picture. This is not some new third calendar but the same sabbatical calendar attested elsewhere with 1,260 monthly days with an additional 4 epagomenal days to complete the year. As described in the Book of Luminaries:
"The sun comes in through a door and rises for thirty days together with the captains of a thousand of the orders of the stars, together with the four which are added to determine the intervals between the four parts of the year ... Truly, they are recorded forever: one in the first gate, one in the third, one in the fourth, and one in the sixth. The year is completed in three hundred and sixty-four days... The four captains which distinguish the four seasons of the year enter first; after them enter the twelve captains of the orders which distinguish the months; and three hundred and sixty captains which divide the days, and finally enter the four epagomenal days, the captains which divide between the four seasons of the year" (82:4-11).
So the year is construed here as divided first into four seasons, and then within those four seasons are placed twelve months, and then 360 days are placed within those twelve months. Finally, in between the seasons are placed the four epagomenal days in order to divide the seasons of the year. This completes the year in 364 days. The Egyptian civil calendar was somewhat similar, with 12 months consisting of 360 days, and with the 5 epagomenal days saved to the end of the year. The Jewish schematic calendar dropped one of these 5 days in order to make the calendar sabbatical (364 - 7 = 52 weeks in a year), and then interspersed the four days throughout the year as the two solstices and equinoxes that herald the seasons. So in short, a duration of seven years would contain 84 months with a total of 2,520 days but the span of time would actually contain 28 additional epagomenal days. There is an additional question of intercalation, of whether there were additional weeks added into the calendar to adjust for the missing day. Scholars disagree on this question, but the evidence suggests that the calendar was not intercalated and thus wandered across the seasons like the Muslim lunar calendar (hence the distinct Enochic complaint about the seasons coming at the wrong times).