That is the limit imposed by Heisenberg's famous uncertaintyprinciple (which states that the precision of a time measurement is limited by the precision of a corresponding energy measurement).
Heisenberg's uncertaintyprinciple, one of the central tenets of quantum mechanics, maintains that you cannot pin down all the physical properties of a particle at the same time.
Einstein had objected to Werner Heisenberg's uncertaintyprinciple and, in general, to the statistics-based, ultimately random foundation of life Bohr was promoting.