Time flow has been embodied in time-dependent Schroedinger equation
representing one of the foundations of quantum mechanics. Pauli's criticism
(1933) has, however, indicated that the assumptions concerning representation
Hilbert space have led to some contradictions. Many authors have tried to solve
this discrepancy practically without any actual success. The reason may be seen
in that two different problems have been mixed: the problem of Pauli (being
more general and more important) and non-unitarity of exponential phase
operator of linear harmonic oscillator introduced by Dirac, as demonstrated in
1964. The problem will be discussed in a broad historical context and a
solution based on extension of representation Hilbert space will be shown.Comment: 12 pages, Presented at the conference "Irreversible quantum
dynamics", ICTP Trieste, 29.7.-2.8.200
