Ancestry.com has been rolling out an enormously valuable resource: the original vital records of many Massachusetts towns. Searchable, no less! What a great tool for genealogists, I have already located and attached hundreds of entries to my tree and read the originals. Unfortunately, having used the feature for a little bit now, I have discovered an unfortunate issue that will undoubtedly cause thousands of incorrect dates to be posted into people's family trees, which will then live on indefinitely as people pass around and share the wrong date.
The problem is simple: the transcribers for these records were apparently not aware of the calendar change that happened in 1752, and as a result they are misreading the "month" value in thousands and thousands of vital records entries, which many are adding to their family trees and sharing with others online. You see, in America, prior to September 1752, the first month of the year was March, not January. The second month was April. The third was May. The fourth was June. And so on, through the 12th month, which was February. Incidentally, that's why the "Oct" in October means 8th (like Octopus, Octogon): because prior to 1752, October was the 8th month, not the 10th month. And the "Dec" in December means 10 (like decimal, decagon, decathlon).
Take a look at this example below, snipped off the Ancestry.com website. The death record of John Farrington at Dedham (first entry on the page) is recorded as "27:4:76". This translates to "27th day of the 4th month of 76." Which the Ancestry.com transcriber dutifully transcribed as "April", when actually the 4th month is June. (We can ignore the transcriber's other mistake, "24" when it is actually "27", for the sake of this discussion, since that is a human error and not a calendar misunderstanding).
This is not just a one-off situation, it appears the entire Dedham records were done incorrectly, so practically every date in the entire VRs is wrong. The entire Springfield VRs appear to have been done incorrectly. The records of Cambridge are likewise all messed up.
Moral of the story: Primary source records are great, but only if you can read them correctly! If you get a vital records date off an Ancestry.com's Massachusetts Vital Record collection, you should read the original record yourself, and see if they made this mistake with your ancestor too.