[Tzfat]

The biblical year (lunar, adjusted for seasons) is on average 365.25 days long, but the average is over a 19 year cycle (lunar). Using the Hillel II calendar (which corresponds to, and carries forward, known dates from First Century and earlier) makes for very interesting "7 year" observations! Because the dates each year can be as close as 354 days apart.

Although the Yovel (Jubilee) is unknown, just like the weekly Sabbath, Judaism has accurately tracked the Sh'mittah year (7th year of rest for the Land) for over 2,500 years. We got in trouble the last time we forgot to count those... We are currently half way through the Sh'mittah cycle.