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TryJulianDateToDateTime

Function TryJulianDateToDateTime(const AValue: Double;
  var ADateTime: TDateTime): Boolean

Example​

procedure ScriptEvent(var Value: variant);
var
  EncodedValue: TDateTime;
begin
  if TryJulianDateToDateTime(2451545.0, EncodedValue) then
    Value := EncodedValue; // 1 January 2000 at 12:00
end;

Usage​

TryJulianDateToDateTime tries to convert an astronomical Julian date into an encoded Gregorian date and millisecond time.

Parameters

NameTypeDescription
AValueDouble, constJulian date number including its noon-based fractional day.
ADateTimeTDateTime, varReceives the converted value when the calendar part is encodable.

Returns

True when the derived Gregorian year/month/day can be encoded; otherwise normally False. Numeric and post-encode failures are not comprehensively caught.

Behaviour

  • 2451545.0 maps to 1 January 2000 at 12:00.
  • Adding 0.5 advances twelve hours to the following midnight.
  • Output is an untagged calendar value; the routine does not retain an astronomical time scale such as UTC or TT.

Errors

Unencodable calendar fields normally return False. Numeric conversion, overflow or timestamp errors can still propagate.

Additional Technical Info​

TryJulianDateToDateTime converts an astronomical Julian date number to Delphi's proleptic-Gregorian TDateTime. Julian integral-day boundaries occur at noon, and the conversion reduces the fractional day to millisecond resolution.

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.TryJulianDateToDateTime. The terminal:

  1. applies Trunc and integer Gregorian-calendar conversion arithmetic;
  2. casts the derived fields to Word and calls TryEncodeDate;
  3. converts Abs(Frac(AValue)) to a millisecond-within-day value;
  4. subtracts 12 hours for the Julian noon boundary; and
  5. adds the fractional milliseconds with DateUtils timestamp arithmetic.

Edge cases and quirks

  • The result is limited by Delphi's encodable years 1..9999. An unencodable derived date normally returns False without assigning ADateTime.
  • The Try contract covers the date-encode decision, not every floating/integer operation. NaN, infinity or extreme magnitudes can raise during Trunc, integer arithmetic or later timestamp increments instead of returning False.
  • Fractional input is quantised through MilliSecondOfTheDay; sub-millisecond detail is lost and binary floating-point approximation applies.
  • The implementation uses the absolute fractional magnitude. Negative/extreme Julian values should not be extrapolated without confirming the supported convention and range.
  • Near the supported date boundaries, the subsequent 12-hour and millisecond adjustments are not wrapped in a second Boolean validation.

Side effects

May write ADateTime; no shared state or I/O.

Performance and concurrency

Constant-time floating-point, integer and timestamp arithmetic with no shared mutable state.

Related entries

External references

Created 2026-07-15