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JulianDateToDateTime

Function JulianDateToDateTime(const AValue: Double): TDateTime

Example​

procedure ScriptEvent(var Value: variant);
begin
  Value := JulianDateToDateTime(2451545.0); // 1 January 2000 at 12:00
end;

Usage​

JulianDateToDateTime converts a Julian date number with its noon-based fractional day to a millisecond-resolution TDateTime.

Parameters

NameTypeDescription
AValueDouble, constJulian date number, including an optional fractional day measured from the Julian noon boundary.

Returns

The equivalent untagged TDateTime, rounded to the millisecond resolution used by the conversion path.

Behaviour

  • 2451545.0 maps to 1 January 2000 at 12:00.
  • Adding 0.5 advances 12 encoded hours and therefore reaches the next midnight.
  • The result contains no timezone or time-scale metadata. The routine does not decide whether the input represents UTC, TT or another astronomical convention.
  • Calendar output uses Velox's supported proleptic Gregorian rules rather than a historical regional calendar switch.

Errors

An invalid, non-finite or unsupported Julian date can raise EConvertError, a floating-point exception or a downstream date/timestamp range exception. Velox does not translate those failures.

Additional Technical Info​

JulianDateToDateTime converts a numeric Julian date into Delphi's proleptic-Gregorian TDateTime representation. Julian dates begin at noon, so an integral Julian date maps to 12:00 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.JulianDateToDateTime. Its conversion terminal:

  1. takes the integral Julian-day portion with Trunc;
  2. derives a proleptic-Gregorian year, month and day and attempts to encode that date;
  3. converts the absolute fractional portion to a millisecond-within-day value;
  4. subtracts 12 hours to align the Julian noon boundary with Delphi's midnight boundary; and
  5. adds the milliseconds through DateUtils timestamp arithmetic.

If the calendar date cannot be encoded, the routine raises the DateUtils invalid-Julian-date conversion error.

Edge cases and quirks

  • The supported result is limited to Delphi's encodable calendar years 1 through 9999. Julian values outside that effective range raise rather than returning a sentinel.
  • Fractional days are reduced to millisecond resolution by the helper path and are subject to normal binary floating-point approximation.
  • The conversion uses Trunc and the absolute fractional magnitude. Do not extrapolate the result for negative or extreme Julian numbers without checking that the desired date lies in the supported calendar range and convention.
  • An integral value is noon, not midnight. Forgetting this 12-hour offset is the most common semantic error.
  • The returned TDateTime is untagged. Formatting it as local time does not make the astronomical input local.

Side effects

None.

Performance and concurrency

Constant-time numeric and calendar conversion with no external I/O or shared mutable state.

Remarks

Record the astronomical scale and timezone convention alongside the input in the surrounding configuration. This function performs representation conversion only.

Related entries

  • IncMilliSecond is used by the terminal adjustment and establishes millisecond quantisation.
  • IsValidDate documents Delphi's supported Gregorian component range.

External references

Created 2026-07-15