Because a reader unfamiliar with the double-dating of our early calendar might misinterpret the date of Zacharys sailing from Weymouth, England on 20 Mar 1634/5 as an uncertainty as to the year, I have included the following brief explanation.
The calendar that Julius Caesar devised for the Romans in 46 B.C. was based on a year of 365.25 days (extra day added every four years) whereas the actual astronomical time was 365.2422 days. This difference only amounted to 11 minutes 14 seconds per year but in 125 years that equalled a whole day. By 1582 the Julian Calendar had accrued an error of 10 days which threw off the date of Easter. Pope Gregory III, head of the Roman Catholic Church, ordered the ten days dropped, the leap year day omitted every 400 years and changed New Years Day from 25th of March to the 1st of January. But in 1582 England, not being a Catholic country, did not accept the new Gregorian calendar. During the next 170 years England and later America began using two calendars---the ecclesiastical Julian calendar with its New Years Day on 25 March (O.S. old style) and the historical Gregorian calendar with its new year beginning on 1 January (N.S. = new style). In 1752 when the error in the Julian calendar amounted to eleven days, England converted to the Gregorian calendar, as had the rest of Europe, by eliminating the eleven days between the 2nd and 14th of September (e.g. the date 7 September 1752 does not exist). With the change to the new calendar, New Years day moved from 25 March to 1 January. This meant that the months of January, February & March (through the 25th) in the years 1582-1752 have a double year. Thus the date that Zachary sailed was a firm date: 20 Mar 1634/5---the year was 1634 on the Julian calendar and 1635 on the Gregorian. The date of his arrival in Boston in May 1635 has only one year because it is after the Julian New Years Day of 25 March.