1. Let's clarify the expression $t - \tau$ in the context of problems involving time variables. 2. Typically, $t$ represents the current time or observation time. 3. The variable $\tau$ often represents a delay, lag, or a previous time instant. 4. The expression $t - \tau$ calculates the difference between the current time and the delayed time. 5. This difference measures how much time has elapsed since the event at time $\tau$. 6. For example, if $t = 10$ seconds and $\tau = 7$ seconds, then $t - \tau = 3$ seconds, meaning 3 seconds have passed since the event at $\tau$. 7. Understanding this helps in analyzing systems with delays or memory effects, such as in physics or signal processing. 8. So, $t - \tau$ is simply the elapsed time between two time points, which is why it is expressed as the difference of these two variables.