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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 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.