It is stored as a Double variable, with the date as the integral part, and time as fractional part. The date is stored as the number of days since 30 Dec 1899. It really should be 31 Dec is.
It appears that the reason for Delphi starting at 30 Dec 1899 is to make it as compatible as possible with Excel while at the same time not adopting Excel's incorrectness about dates. Historically, Excel played second fiddle to Lotus 1-2-3. Lotus (which may have got this error from Visicalc) incorrectly considered 1900 to be a leap year hence a value of 60 gives you 29 Feb 1900 in Excel but is interpreted as 28 Feb 1900 in Delphi due to Delphi starting 1 day before. From the 01 Mar 1901 the two date systems give the same result for a given number.
01 Jan 1900 has a days value of 2.
Because TDateTime is actually a double, you can perform calculations on it as if it were a number. This is useful for calculations such as the difference between two dates.
No local time information is held with TDateTime - just the day and time values.