On time-interval transformations in special relativity

We revisit the problem of the Lorentz transformation of time-intervals in special relativity. We base our discussion on the time-interval transformation formula $c\Delta t' = \gamma (c\Delta t - \vec{\beta} \cdot \Delta \vec{r})$ in which $\Delta t'$ and $\Delta t$ are the time-intervals between a given pair of events, in two inertial frames $S$ and $S'$ connected by an general boost. We observe that the Einstein time-dilation-formula, the Doppler formula and the relativity of simultaneity, all follow when one the frames in the time-interval transformation formula is chosen as the canonical frame of the underlying event-pair. We also discuss the interesting special case $\Delta t' = \gamma \Delta t$ of the time-interval transformation formula obtained by setting $\vec{\beta} \cdot \Delta \vec{r}=0$ in it and argue why it is really \textbf{not} the Einstein time-dilation formula. Finally, we present some examples which involve material particles instead of light rays, and highlight the utility of time-interval transformation formula as a calculational tool in the class room.