One of the requirements necessary in the GPS (Genealogical Proof Standard) is accurate dates.
How does one find accurate dates when there is almost no information to go on.
The simplest method is to calculate a time period in which the event was to have happened. Say that two people married and have a child named James, born 23 September 1792. Assuming that James is the only known child of the couple, we can make a guess that the parents Abby and John, were in their twenties or thirties at the time. If James were the first child, then we subtract the minimum ages for marriage at the time and go from there.
First, we find that the minimum age for men and women to marry was eighteen. If John, the father were eighteen when he married Abby the latest date for Abby’s birth year would be 1774. If James were not the first child, then we could add, say thirty years’ time to the calculation to account for a large family in between John and Abby’s marriage and James’ birth. The earliest date for Abby’s birth year would be 1762.
We also have to allow a reasonable time for other life events, such as military service, and the fact a pregnancy may have started later than the marriage date. So we add, say a year to the calculations. Thus, we can say that John would have been born as early as 1761 and as late as 1773 to be old enough to have James in 1792.
A proper way of stating these dates in writing about the family is to use “say” or “perhaps” dates, which are less specific than exact date calculations. We would write “Abby was born, say 1761.” Or, we could write “John, the father, was born perhaps as early as 1773.” On the other hand, even, “Abby was born circa 1761-1773.”
One easy way to do this is to estimate the dates using a spreadsheet. I use OpenOffice.org’s Calc for most of my work and it is easy to set up the spreadsheet in it. These calculations also work similarly in Microsoft Excel.
For a more detailed discussion of date estimation, I refer you to Derek Harland’s Genealogical Research Standards, Salt Lake City, Utah: Bookcraft, Inc., 1963, especially chapter six. You can find copies via Bookfinder.com or Used.Addall.com.