The issue I keep running into is this: the vital records for a locality, such as Dedham, Massachusetts, list an event as "the 15th day of the 2nd month."
Example: my 9th great-grandmother Eliony Lumber was married to Edward Hawes on the 15th day of the 2nd month of 1648 in Dedham, Norfolk, Masschusetts.(1)
So that means February 15, 1648, correct? Actually the correct date is April 15, 1648.
Keep in mind that under the Julian Calendar, the first day of the new year, for church and legal purposes was March 25th, also known as Lady Day or the Feast of the Annunciation. So the 1st month would be March, and the 2nd month would be April.
In each country you should remember the cutoff dates for the switch from the Julian Calendar to the Gregorian Calendar.
There are some great resources out on the Web, especially a recent post by Carolyn L. Barkley over at GenelogyandFamilyHistory.com.
Check out The Perpetual Calendar for precise dates when certain places made the switch.
(1) Vital Records of Dedham, Massachusetts, 1635 - 1845, Online Database: NewEnglandAncestors.org, New England Historic Genealogical Society, 2007,