計算該日期當月的天數

自訂函數做法如下：function LastDay(xDate: TDate): Word;
var
Y, M , D: Word;
const aMonthLastDay: array[1..12] of word
=(31,28,31,30,31,30,31,31,30,31,30,31);
begin
DecodeDate(xDate, Y, M, D);
if Y mod 4 = 0 then
aMonthLastDay[2] := 29
else
aMonthLastDay[2] := 28;
result := aMonthLastDay[M];
end;