Anyone who can trace their ancestors back to the 1700s needs to understand the workings of the old Julian calendar. It is the predecessor to the modern (Gregorian) calendar that we use today. Dates recorded in the old Julian calendar are different than dates used today. Even the year end was different (25 March instead of 31 December).
The GenealogyInTime website recently published a helpful article to aid in understanding the difference between the two calendars. You can read the article at http://goo.gl/r1ysf