如果把两个日期时间类型的差乘24乘60乘60，结果刚好是“整数”。
TDateTime is a used by the date and time routines to hold date and time values.
Unit
System
type TDateTime = type Double;
Description
Most VCL objects represent date and time values using the TDateTime type. The
integral part of a TDateTime value is the number of days that have passed since
12/30/1899. The fractional part of a TDateTime value is fraction of a 24 hour
day that has elapsed.
Following are some examples of TDateTime values and their corresponding dates
and times:
0 12/30/1899 12:00 am
2.75 1/1/1900 6:00 pm
-1.25 12/29/1899 6:00 am
35065 1/1/1996 12:00 am
To find the fractional number of days between two dates, simply subtract the
two values. Likewise, to increment a date and time value by a certain fractional
number of days, simply add the fractional number to the date and time value.
Note: Delphi 1.0 calculated the date from year 1 instead of from 1899. To
convert a Delphi 1.0 date to a Delphi 2.0 date, subtract 693594.0 from
the Delphi 1.0 date. The date format changed to be more compatible with
OLE 2.0 Automation.
