  Quantum mechanics rests on the assumption that time is a classical variable.
As such, classical time is assumed to be measurable with infinite accuracy.
However, all real clocks are subject to quantum fluctuations, which leads to
the existence of a nonzero uncertainty in the time variable. The existence of a
quantum of time modifies the Heisenberg evolution equation for observables. In
this letter we propose and analyse a generalisation of Heisenberg's equation
for observables evolving in real time (the time variable measured by real
clocks), that takes the existence of a quantum of time into account. This
generalisation of Heisenberg's equation turns out to be a delay-differential
equation.
