I sometimes read email from Whipple relatives who might not realize the idiosyncrasies of how approximate dates appear in the Whipple Genweb. I offer this post to (hopefully) clarify approximate dates that appear there.
Abbreviation for about. When "Abt" precedes a date, it generally means that the event likely occurred within a year or two of that date.
This is common when censuses give an age but no birth year--I just subtract the age from the census year and use "Abt [year]" as the birth date. When I find actual vital records, I generally update the dates to be more exact.
Abbreviation for after. It can sometimes mean "a long time after."
It probably appears most often when I don't know a death date, but found the individual listed in a census. I will often use "Aft [year]", where "[year]" is the year of the census that lists the individual.
To illustrate: I might find a person mentioned in a 1910 census, but don't have time to do additional research (at that moment). I might, then, record the death date as "Aft 1910" (even though the person might have actually lived until, for example, 1950 or 1960).
This also occurs when an obituary lists survivors: I will use the death date of the person who died (preceded by "Aft") for the survivors.
"Bef" and "Bet"
Abbreviations for "Before" and "Between". These (like "Aft") can be very inexact. (See above)
In just about every case, "Abt" dates are the closest to the actual date. Other dates can be far from the actual dates.