Delphi in a Nutshell

Name

TDateTime Type

Syntax

type TDateTime = type Double;

Description

The TDateTime type stores a date and time as the number of days since the start of the day on December 30, 1899, which is the standard used in OLE automation. The integer part is the number of days, and the fractional part specifies the time of day.

Tips and Tricks

• TDateTime does not store a time zone, so if you want to record a date and time in a file or database, you should convert the local time to UTC (Coordinated Universal Time). When loading the date and time from the file or database, convert back to local time. That lets users work in different time zones without running into problems.

• You can count the number of days between two TDateTime values by subtracting, and converting the result to an integer. Whether you round off or truncate depends on the circumstances. Do you want to count the number of “days” between 1:00 A.M. on March 3 to 11:00 P.M. on March 4 as one day (truncate) or two days (round off)?

• Unix represents a date and time as the number of seconds since midnight January 1, 1970 UTC. If a file or database contains a Unix time, you can convert it to a Delphi TDateTime by dividing by SecsPerDay and then adding 25569. That gives you the UTC date and time; you must convert this to a local date and time before using it in most Delphi programs.

Example

// Convert a local time to UTC by adding the time zone bias. // To convert from UTC back to local, subtract the bias. function LocalToUTC(DateTime: TDateTime): ...