Winter Ends Sunday morning March 20 at 7:33AM. You must be back to the trailhead by then.
From the AMC Four Thousand Foot Club website:
Q. What determines a winter ascent?
A. Many hikers believe that winter starts on Dec 21st and ends on March 20th. Things are not quite so simple.
Trips must begin after the hour and the minute of the beginning of winter (winter solstice), and end before the hour and minute of the end of winter (spring equinox), to satisfy the criteria established by Miriam Underhill, the inventor of the Winter Four-Thousand-Footer Game.
In the US edition of her book, Give me the Hills Miriam Underhill is even more specific, as she writes:
This game was an offshoot, of course, of that very popular game of the Appalachian Mountain Club, Climbing the Four-thousanders, which was set in motion, and such vigorous and enthusiastic motion, in 1958. Our game —"ours" because we were the first to play it— followed right along. As the initiators we set the rules, which concerned the definition of "winter". "Snow on the ground" and other namby-pamby criteria definitely did not count. "Winter" was to be measured exclusively by the calendar. In 1960, for instance, winter began at 3:27 PM on Wednesday, December 21, too late to get up to Crag Camp by daylight.
Notice the use of the word "game". Games have rules, which may well be arbitrary, but if you play a game you should follow the rules. If you do not like the rules, you are free to define your own game, but must clearly differentiate it from the "official" game. Also note the last sentence of the quotation above. The entire trip has to start (and end) during winter. She did not allow the trip up to Crag Camp to start one day before winter, with the final ascent taking place on the first day of winter.
The official times of the Equinoxes and Solstices are available on the US Naval Observatory web site. Note that the times are given in Universal Time (UT). To convert to EST subtract five hours. The following table (using EST) is extracted from that site:
Winter of Start End
2002-2003 Dec. 21, 8:14 PM Mar. 20, 8:00 PM
2003-2004 Dec. 22, 2:04 AM Mar. 20, 1:45 AM
2004-2005 Dec. 21, 7:42 AM Mar. 20, 7:33 AM
Note that while the first day of winter 2002 was December 21st it occurred after dark in 2002 so the first full hikable winter day in 2002-2003 was Dec 22nd. In 2004, December 21st is a full hiking day.
Similarly, except in 2003, March 19th is the last full hikable day of winter, as winter ends too early on the 20th of other years to allow a full day of hiking.