$\Delta t=t_{\text{B}}-t_{\text{A}}$

Note 1 to entry: The term "local duration" is usually used in relation to the situation in which event B is in the absolute future with respect to event A. Then, in any two different inertial frames S, S′, local durations $\Delta t$ and $\Delta t^{\prime }$, mutually being different, i.e. $\Delta t\ne \Delta t^{\prime }$, obey relations $\Delta t>0$ and $\Delta t^{\prime }>0$. If event B is in relative presence, the local duration can be negative, zero or it can even have different signs in different frames S, S′.

Note 2 to entry: The coherent SI unit of local duration is second, s.