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DateDelta

DateDelta = 693594

Example​

function IsDelphiZeroTimestampDate(const TimestampDate: Integer): Boolean;
begin
  Result := TimestampDate = DateDelta;
end;

Usage​

DateDelta defines Velox's 693,594-day offset between TDateTime serial dates and TTimeStamp day numbering.

Additional Technical Info​

DateDelta is registered by the modified PascalScript Classes compiler and matches Delphi System.SysUtils.DateDelta. It is an internal ordinal offset used when converting between a floating TDateTime and the Date member of Delphi's millisecond TTimeStamp record.

For an ordinary nonnegative value, current Delphi DateTimeToTimeStamp stores DateDelta + Trunc(DateTime). Therefore TDateTime(0), whose civil date is 30 December 1899, maps to timestamp Date = 693594. The reverse routine subtracts DateDelta before rebuilding the floating serial.

This is not a duration constant for application scheduling, and it is not the Unix epoch offset. In particular:

  • adding it to a normal TDateTime does not convert local time to UTC;
  • it does not convert seconds or milliseconds;
  • Unix conversion uses a separate 25,569-day basis plus UTC/local handling; and
  • negative Delphi serials require the RTL's sign-aware conversion rather than a simple Trunc(Value) + DateDelta formula.

The installed SysUtils source contains historically awkward comments: the public declaration describes days through 31 December 1899, while the conversion code identifies DateDelta as the timestamp date corresponding to Delphi's zero day, 30 December 1899. Rely on the value and traced conversion routines, not an inferred inclusive/exclusive calendar count.

Use the public date/time conversion functions unless an external binary/API contract explicitly requires a Delphi TTimeStamp.Date ordinal. The example only identifies that internal zero-day ordinal; it does not convert a complete timestamp and was not executed.

Related Code Library entries

External references

Created 2026-07-15