Dt 30683 9 Across

[barmy]

Longer and shorter periods reversed on Greek letter 5 letters.

I am working to the answer being a Greek letter ending in a but there are 7 and I can't fit any into the down clue. Am I on the wrong track, any help appreciated 

Ta 

[bhg481]

omega if you accept mo = month

[Neveracrossword]

Omega, Greek letter - that has Age, a long period, and Mo, a short period, both reversed.

[bhg481]

You could also argue mo = moment, as in "just a mo"

[Neveracrossword]

Barmy, glad I could help. Mo, a short period of time, pops up fairly often in the puzzles I do.