Jam Tomorrow Meaning

(idiomatic) Promised benefits that never arrive.

Example: 1930, John Maynard Keynes, “Economic Possibilities for Our Grandchildren”:
  The “purposive” man ... does not love his cat, but his cat’s kittens; nor, in truth, the kittens, but only the kittens’ kittens, and so on forward forever to the end of cat-dom. For him jam is not jam unless it is a case of jam to-morrow and never jam to-day.
1961, Bulletin of the Atomic Scientists (volume 17, number 8, October 1961)
  Yet they've proved that common men can show astonishing fortitude in chasing jam tomorrow.
1978, Eileen M. Byrne, Women and education
  It always seems to be a problem to be dealt with when resources (later) permit. Jam tomorrow, as usual.