The ancient Jews had several different calendars.
The Jewish lunisolar calendar had 354 days per year (based on alternating 29-day and 30-day months, as determined by observation of the moon and later by known astronomical cycles), which was intercalated with the solar cycle but the insertion of an extra month Adar II every several years. This calendar is used in much of the OT, especially those with named months.
The Jewish solar calendar had 360 monthly days per year (based on four seasons consisting of three 30-day months), plus 4 epagomenal days inserted between the seasons, namely the spring equinox, the summer solstice, the autumn equinox, and the winter solstice. In later Jewish texts (cf. Jubilees, the later sections of the Book of Luminaries in 1 Enoch, the Qumran calendrical texts), these four days came to be included within the months, giving a total of 364 days in the months. This calendar was used in the Zadokite/Sadducee cultus and in the Essene community, and is used (in its original form) in Daniel, the Priestly Code, and other priestly texts in the OT. This was a favored religious calendar by the priests because it was inherently sabbatical (364 is divisible by 7, yielding 52 weeks), allowing the festivals and sabbaths to fall on the same day of the week every year. The Christian reckoning of Easter and Holy Week is a remnant of this.
The Society fails to understand the solar calendar used in Daniel when they calculate the "Gentile Times". They do not realize that the yearly computation (since they identity a "time" = a year) included not only the 360 monthly days but also the 4 equinoxes and solstices. Only when reckoning the year by the months (such as the 42 months of Revelation 11:2-3) were the epagomenal days omitted by the Zadokite calendar (while the Essene and Jubilees calendar included them).