In the early 4th Century, Greenwich was an insignificant village in an far flung part of the Empire, 1,500 miles from Nicaea, and it would be 1,500 years before it became the internationally-accepted standard for time. Local solar time in Nicaea is almost exactly 2 hours ahead of Greenwich, and Jerusalem (which is where the events being commemorated happened) is a further 20 minutes ahead - so, where it mattered, the Equinox was at 02:19 this morning (March 21st).