TDateTime represents a dateandtime value in the Delphi language.
TDateTime = type Double;
type Double TDateTime;
In Delphi, TDateTime is a type that maps to a Double. In C++, the TDateTime class corresponds to the Delphi TDateTime type.
The integral part of a Delphi TDateTime value is the number of days that have passed since 12/30/1899. The fractional part of the 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, unless one of the TDateTime values is negative. Similarly, to increment a date and time value by a certain fractional number of days, add the fractional number to the date and time value if the TDateTime value is positive.
When working with negative TDateTime values, computations must handle time portion separately. The fractional part reflects the fraction of a 24hour day without regard to the sign of the TDateTime value. For example, 6:00 am on 12/29/1899 is –1.25, not –1 + 0.25, which would be –0.75. There are no TDateTime values between –1 and 0.
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