means a fictional timekeeping system used in star trek where a decimal number replaces calendar dates, since earth calendars mean nothing in deep space.

from gene roddenberry introduced it in the 1960s original series to sidestep the awkward fact that a starship traveling near light speed and across time zones can't just say 'tuesday.' the writers deliberately kept the numbers vague and non-sequential in tos, which drove continuity nerds mad for decades.