  In an abstract framework, a new concept on time operator, ultra-weak time
operator, is introduced, which is a concept weaker than that of weak time
operator. Theorems on the existence of an ultra-weak time operator are
established. As an application of the theorems, it is shown that Schroedinger
operators H with potentials V obeying suitable conditions, including the
Hamiltonian of the hydrogen atom, have ultra-weak time operators. Moreover, a
class of Borel measurable functions $f$ such that $f(H)$ has an ultra-weak time
operator is found.
