SF primitives that producing the current running time.

Time is global for an SF, so, every constituent SF will use the same global clock. However, when used in combination with switching, the SF switched into will be started at the time of switching, so any reference to localTime or time from that SF will count using the time of switching as the start time.

Take also into account that, because derivative is the derivative of a signal over time, differentiating localTime will always produce the value one (1). If you really, really, really need to know the time delta, and need to abandon the hybrid/FRP abstraction, see iterFrom.