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Contrary to DaysBetween, DaySpan function does not count only whole days. What this means is that it will NOT return 0 as the result for difference between 05/01/2003 23:59:59 and 05/01/2003 23:59:58 - where the actual difference is one *whole* day minus 1 second.

var dtNow, dtBirth : TDateTime;
    fracDaysFromBirth : double;

dtNow := Now;
dtBirth:=EncodeDate(1973, 1, 29);
fracDaysFromBirth := DaySpan(dtNow, dtBirth);

ShowMessage('Zarko Gajic "exists" ' + FloatToStr(fracDaysFromBirth));