= Unit of time
{title2=$\mathrm s$}
A <unit of time> is a reference duration used to express a time interval. Writing $\Delta t=2\,\mathrm{min}=120\,\mathrm s$ 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>.<The International System of Units (SI), ninth edition, SI Brochure>[Section 2.3.1, definition of the second]{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>.
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