In front of a year, this means "about". On this web site it is used exclusively for dates of birth derived from a Census. Where a Census record gives someone's age, the date of birth is obviously calculated by subtracting the age from the Census Date. However, some people will round up their age, others will lie !! Latin "circa". On this web site it is used in front of a date which is accurate to within about one year. For example, tombstones often give the precise date of death and the age. If the death was before the birthday, the resulting subtraction would give a year of birth one year later than the actual year of birth. Also, it would not be known if the family "rounded" the age up or down.
actual bottom time/bottom time. The number of minutes that a diver spends underwater on a particular dive, calculated from beginning of descent to beginning of direct ascent to the surface or a safety stop. Used in calculating the repetitive group designation on a dive table.