A Julian year is the astronomical duration of exactly 365.25 days, with each day here equal to 86400 seconds:It is a fixed unit of time, not a claim that every calendar year or every orbit of Earth has precisely that duration. In particular, an ordinary 365-day calendar year is shorter by six hours.[ref]
Measuring the Universe by the International Astronomical Union. Defining a year, 365.25 days and 31.5576 million seconds
The fixed convention makes light-year conversions unambiguous: multiplying one Julian year by the vacuum speed of light gives exactly . Similarly a rate of two milliarcseconds per Julian year corresponds to an angular change of ten milliarcseconds over five such years. A calendar-based rate needs its own time-scale definition before such conversions are exact.
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