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DateTimeToJulianDate

Function DateTimeToJulianDate(const AValue: TDateTime): Double

Example​

procedure ScriptEvent(var Value: variant);
var
  Timestamp: TDateTime;
begin
  Timestamp := EncodeDate(2000, 1, 1) + EncodeTime(12, 0, 0, 0);
  Value := DateTimeToJulianDate(Timestamp); // 2451545.0
end;

Usage​

DateTimeToJulianDate converts a TDateTime value to a Julian date whose day boundary is noon.

Parameters

NameTypeDescription
AValueTDateTime, constGregorian date/time to convert. The value itself supplies no timezone.

Returns

A Double Julian date. For example, 1 January 2000 at 00:00 is 2451544.5; noon is 2451545.0.

Behaviour

The half-day offset is intrinsic to the Julian convention: a civil midnight has a .5 fractional result and civil noon is an integer boundary. The time fraction is carried to the precision available in the input Double; it is not restricted to whole seconds by this routine.

Errors

No error handler is added by Velox. Valid Velox dates convert directly; failures from decoding or unsupported floating values propagate.

Additional Technical Info​

DateTimeToJulianDate converts a Delphi TDateTime calendar value into the continuous Julian date convention used in astronomy and related calculations. The result includes a fractional day and changes its integer day number at noon rather than midnight.

The example is fictional, deterministic and source-reviewed. It was not executed by the documentation workflow.

Implementation

The DateUtils import binds directly to Delphi 37.0 System.DateUtils.DateTimeToJulianDate. The terminal decodes the Gregorian year, month and day, applies the integer Gregorian-to-Julian calendar formula, subtracts 32075.5, then adds Abs(Frac(AValue)) as the time fraction.

Edge cases and quirks

  • The result describes the supplied calendar fields only. It is not automatically Greenwich/UTC merely because Julian dates are commonly used as universal time coordinates; normalise the input before conversion when that meaning is required.
  • The terminal uses Abs(Frac(AValue)) so Delphi's negative TDateTime time convention contributes a positive time-of-day fraction.
  • Floating-point results can contain representation error. Compare or format them with an appropriate tolerance/precision rather than assuming exact decimal equality.
  • Invalid raw TDateTime values can decode to zero fields or overflow intermediate calendar arithmetic. The script binding performs no validation.
  • Free Pascal documents the same conversion concept, but Velox runs the installed Delphi algorithm.

Side effects

None.

Performance and concurrency

Constant-time integer/floating arithmetic with no allocation or shared state.

Related entries

External references

Created 2026-07-15