  A unique description avoiding confusion is presented for all flavor
oscillation experiments in which particles of a definite flavor are emitted
from a localized source. The probability for finding a particle with the wrong
flavor must vanish at the position of the source for all times. This condition
requires flavor-time and flavor-energy factorizations which determine uniquely
the flavor mixture observed at a detector in the oscillation region; i.e. where
the overlaps between the wave packets for different mass eigenstates are almost
complete. Oscillation periods calculated for ``gedanken'' time-measurement
experiments are shown to give the correct measured oscillation wave length in
space when multiplied by the group velocity. Examples of neutrinos propagation
in a weak field and in a gravitational field are given. In these cases the
relative phase is modified differently for measurements in space and time.
Energy-momentum (frequency-wave number) and space-time descriptions are
complementary, equally valid and give the same results. The two identical phase
shifts obtained describe the same physics; adding them together to get a factor
of two is double counting.
