What does that mean?
Let me give an example of an "Old Style" date and explain what is going on. This appeared in a newspaper article I found that really struck me—an obituary for Hannah Lyman, published in the Hampshire Gazette (Northampton, Massachusetts), 21 March 1832, page 3.
First of all, what a great story this obituary tells. Hannah Lyman experienced the earthquake of 1755 and it so affected her that she retold the story all her life. If you've ever experienced an earthquake, it really gets your attention. The noise, the shaking—all so frightening—you try to orient yourself to what's happening—what it means.
This was the famous Cape Ann earthquake of 1755. According to Wikipedia this was "the largest earthquake in the history of Massachusetts." Cape Ann and Boston felt the brunt of the earthquake's aftermath; however hundreds of homes and buildings throughout the state of Massachusetts were also damaged. Northampton, Massachusetts, is 142 miles from Cape Ann, Massachusetts.
Hannah remembered the exact date of the earthquake: 18 November 1755. She was the oldest person in Northampton, Massachusetts, in early 1832—and she vividly remembered the terrifying earthquake that happened way back when she was 12 years old—almost 77 years ago. There was no quick Internet where she could quickly look it up. It stuck in her mind, just like her date of birth.
But look at how her date of birth is reported in her obituary: "Sept. 15, 1743, Old Style."
Yes, that was a common clarification added to dates around the time of the adoption of the "Gregorian" calendar that we still use today. The American colonies adopted the Gregorian calendar in 1752, shifting the start of the year from March 1st to January 1st.
It became standard when referring to dates before 1752 to distinguish between Old Style (O.S.) and New Style (N.S.).
For example, Thomas Jefferson was also born in 1743, like Hannah, and he asked that the date of birth on his gravestone be clearly indicated by including the designation O.S.
I am struck that an obituary published in 1832 can give us such memorable information about our ancestors. Hannah experienced the Cape Ann earthquake, witnessed the American Revolution—and even had to deal with the hassle of the world changing the calendar itself!
She lived through interesting days—and we never would have known about some of her experiences if they had not been recorded in the old newspapers.
So keep an eye out for the Old Style and New Style date designation when you are doing your family history research—it may help you accurately calculate the birth and death dates of your ancestors.