Those who study history and genealogy realize that dates are not always exactly known. Many times, historical records do not exist or only give partial information. Sometimes, we must make an educated guess at the date an event occurred. When we do that, it is important to indicate that the chosen date is not exact. The most common way to do that is to use the abbreviation ca.
This abbreviation stands for the Latin word “circa” which means “approximately.” This is the perfect word to communicate that we have done our best to determine a correct date, but do not have the ability to guarantee its exactness.
In the study of genealogy, this is often necessary because we usually trace family trees by moving backwards in time. You may start with a relative whom you know is about 80 years old according to a census record in 1940 and from there deduce that his birth happened around 1860. But, census records are not always 100% accurate. So, until you find his actual birth record, you might record his birth year as ca. 1860.
It is common to put the abbreviation before years, but less common before months and actual days. This is because by the time we get down to listing months and days, we are usually quite certain of the year. Adding the abbreviation ca in front of a complete date would indicate uncertainty about the entire date including the year. That said, it can be done if necessary.