If the Qumran community had survived to the present, they would have gained 6 years on the Roman calendar...so they would have celebrated Passover, Rosh Hashanah, Yom Kippur, and Hanukkah six extra times.
Looks like, as I read it, they had an extra day inserted annually; that would cover it, as long as they added a second leap day every fourth year, or as necessary, when it started getting out of whack.
52 7 day weeks, and an extra non calendrical “year day”, with the occasional “double year day” tossed in.
sort of like: Sunday, March 19th; Monday, March 20th; Equinox Day/New Year Day; (Leap Day, if needed); Tuesday, March 21st; ...