In Appendix D Tolkien tells us: "The Fourth Age was held to have begun with the departure of Master Elrond, which took place in September 3012;
Would that be September 3021... when he sailed from the Havens?
Using the Appendix B 3021 date for this entry, I used what I found that I thought related to that when Frodo and Sam met up with Elrond, Galadriel, Bilbo as they rode to the Grey Havens.
With both the Shire Reckoning and the New Reckoning of King Elessar there is a gap between the end of the Third Age and the start of the Fourth, but the gap is of different length in the two calendars.
Do you think the gap was the time elapsed when the ship with all the Ring-bearers was in transit? ;)