Of the day-count calendar astronomers keep — days numbered in one unbroken line since antiquity, no months in the way.
Latin Iulius, the name of a Roman family; the Julian calendar reformed under that name in 46 BC gives the adjective its common sense, though the Julian Day astronomers count from 4713 BC was named in 1583 for a different Julius entirely — the father of the scholar who devised it.
A count of days running in one unbroken line since antiquity, with no months in the way to interrupt the counting.