When your research takes you back in time to the 1700s, there is a little problem you need to watch out for. In 1752, Britain and its colonies switched from the Julian calendar to the Gregorian calendar. The old Julian calendar was based on an ecclesiastical (church) system of dates. March 25 was the first day of the new year under this system. The church picked this date because it is nine months before Dec. 25, the established date of Jesus' birth. This day was celebrated as the Feast of the Annunciation which the church believed to be the day that Mary was visited by the angel Gabriel and told she would be the mother of the Messiah.
When you have a series of events that took place before 1752 you can easily make a mistake in the sequencing of these events. It will appear that a certain event took place before another when it in fact took place after. Genealogists handle this is by using a double dating system.
When you have events that occurred between Jan. 1 and March 24 (inclusive) prior to the calendar change, you will need to record both the Julian and the Gregorian year.
Let's say that you are looking at probate records. You find your ancestor's will drawn in November of 1718, but it was probated in February of 1718. On the surface, it appears some error has been made. It is impossible for a will to be probated eight months before it was actually written. Under the Julian calendar the new year would not have begun until March 25. So under that old system February is actually later in the same year as November. Instead, we would record the dates like this:
John Doe's will drawn Nov 1718
John Doe's will probated Mar 1718/9
Now we can see that the will was actually probated 4 months after it was drawn.
There are a few special situations you need to be aware of. Some of the colonies and/or certain groups of people switched to the new calendar earlier or later than others. Two of the most notable examples are the Dutch of New Netherland who switched prior to the colonization of the New World and the Quakers who never used the ecclesiastical calendar in the first place. You will just need to pay attention to the dates closely. It is usually easy to figure out whether or not the old or new calendar was being used. You will also find some records double dated that shouldn't be. For example, you might see a date such as May 15, 1715/6. Since this date is after March 24th the correct date would simply be May 15, 1716. Another interesting thing you might see before 1752 is something like this: 8ber and 9ber. This actually means October and November and not August and September.
The good news is that all of the available genealogical computer programs will convert your dates automatically making it virtually foolproof. You need only watch for the special situations that might come up.