  We draw a picture of physical systems that allows us to recognize what is
this thing called "time" by requiring consistency not only with our notion of
time but also with the way time enters the fundamental laws of Physics,
independently of one using a classical or a quantum description. Elements of
the picture are two non-interacting and yet entangled quantum systems, one of
which acting as a clock, and the other one doomed to evolve. The setting is
based on the so called "Page and Wootters (PaW) mechanism", and updates, with
tools from Lie-Group and large-$N$ quantum approaches. The overall scheme is
quantum, but the theoretical framework allows us to take the classical limit,
either of the clock only, or of the clock and the evolving system altogether;
we thus derive the Schr\"odinger equation in the first case, and the Hamilton
equations of motion in the second one. Suggestions about possible links with
general relativity and gravity are also put forward.
