From AMS Glossary
A system of local time classification, differing from coordinated universal time (UTC) in steps of 1 hour per 15° of longitude.
The individual time zones are categorized by the letters A, B, C, etc. (omitting J) for areas centered on 15°E, 30°E, 45°E, etc. and by the letters N, 0, P, etc. for areas centered on 15°W, 30°W, 45°W, etc., respectively. UTC is, in this system, designated as Z time.