There was no such thing as a "prophetic calendar". The texts in Daniel (e.g. 7:25, 10:4, 12:11-12; note that 4:16, 32 makes no allusion to calendrical details like the other texts) and Revelation (11:2-3, 12:6, 14) instead reflect the priestly solar calendar which was followed by the Sadducees, the Essenes, and the authors of the Book of Luminaries of 1 Enoch and Jubilees. It had twelve months of 30 days each, which were arranged in four seasons of equal length; thus Revelation 11:2-3 correctly states that 1,260 days equalled 42 months. The actual length of the year however was 364 days (instead of 360 days) since in between the seasons were the intercalary markers of the seasons: the two equinoxes and the two solstices. It was only in the latter history of the calendar that these epigomenal days became counted within the months. We know that the year was not literally 360 days in length because it was sabbatical in nature (with the festivals and sabbaths falling on the same day of the week each year), and 364 is evenly divisible by 7, producing 52 weeks a year. Moreover, sabbatical years would also have an intercalary week to make good the extra day omitted by the 364-day scheme, making it essentially the same length as the Julian calendar.
Ignorant of the actual mechanics of the calendar, the Society is actually correct that the 1,260 days of Revelation (and by extension their 2,520 days) include extra days when reckoned sequentially, for this time period does indeed skip over the non-monthly epagomenal days. That is to say, 2,520 years would not consist of 907,200 days as if the epagomenal days did not exist. They do exist -- they are necessary for the sabbatical compenent of the calendar to work. The Society's error instead (aside from the blatant misinterpretation of Daniel 4) is that in computing the "seven times" as a period of consecutive day-years, they failed to take these epagomenal days into account. A seven-year period in the solar calendar amounted to 2,548 consecutive days when reckoned yearly (while only 2,520 of those would be monthly days), and the seventh year -- being sabbatical -- would add an extra week, hence a total of 2,555 days. So instead of 1878 or 1879, and instead of 1914, the Society should have instead calculated a consecutive period of day-years as leading to the date 1949.