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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]

The fixed convention makes light-year conversions unambiguous: multiplying one Julian year by the vacuum speed of light gives exactly $9460730472580800\mathrm{m}$. 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.

A light-year is the length travelled by light in vacuum during one Julian year of exactly 365.25 days. Using $86400$ seconds per day and the exact vacuum speed of light givesIt is therefore approximately $9.46\times 10^{15}$ metres, or 9.46 trillion kilometres. The Julian-year convention matters because a calendar year need not have that duration.^[ref]

For a stationary source four light-years away in an idealized flat spacetime, light takes four Julian years to reach the observer. The distance unit and the propagation time remain different physical quantities. In an expanding universe, present distance, lookback time multiplied by the speed of light, and luminosity distance generally differ, so this simple nearby-source interpretation cannot be applied to every cosmological distance.

A unit of time is a reference duration used to express a time interval. Writing $\Delta t=2\min =120\mathrm{s}$ changes the numerical representation, not the elapsed time in physics. A date labels a point on a time scale; a duration measures separation between two such points.

The SI base unit is the second, defined by fixing the unperturbed ground-state hyperfine transition frequency of the caesium-133 atom to exactly $9192631770\mathrm{Hz}$. A clock realizes that definition by comparing an oscillator with the atomic response; practical imperfections still create measurement uncertainty.^[ref]

A minute is exactly 60 seconds, an hour 3600, and a day used as a duration unit 86400. A calendar day need not coincide with a uniform 86400-second interval on every civil time scale. Specifying the time convention matters when combining clock measurements, astronomical observations and Julian years.