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ModifiedJulianDateToDateTime

Function ModifiedJulianDateToDateTime(const AValue: Double): TDateTime

Example​

procedure ScriptEvent(var Value: variant);
begin
  Value := ModifiedJulianDateToDateTime(60000.0); // 25 February 2023 at midnight
end;

Usage​

ModifiedJulianDateToDateTime converts a Modified Julian date, whose integer boundary is midnight, to a millisecond-resolution TDateTime.

Parameters

NameTypeDescription
AValueDouble, constModified Julian date. The integer portion identifies a midnight boundary and the fractional portion identifies the time since that midnight.

Returns

The corresponding proleptic-Gregorian TDateTime, rounded to Velox's millisecond conversion resolution. The result has no timezone or time-scale tag.

Behaviour

  • Modified Julian date zero represents 17 November 1858 at 00:00:00.000 in the adopted calendar conversion.
  • Adding 1.0 advances one day; adding 0.5 advances twelve hours.
  • The returned calendar fields use Velox's proleptic Gregorian encoder.
  • No UTC, TT or other astronomical time-scale conversion is performed. The numeric scale is interpreted exactly as supplied.

Errors

An unencodable or non-finite input raises EConvertError through the Julian-date Velox. Velox does not catch or translate it.

Usage notes

Do not pass an ordinary Julian date without first accounting for the 2,400,000.5 offset and its noon boundary.

Additional Technical Info​

ModifiedJulianDateToDateTime converts a Modified Julian date number to Delphi's encoded TDateTime representation. An integer Modified Julian date changes at midnight rather than at the noon boundary used by an ordinary Julian date.

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.ModifiedJulianDateToDateTime. It adds the fixed offset 2,400,000.5 and delegates to JulianDateToDateTime. That terminal derives a Gregorian date, shifts the Julian noon boundary to midnight, converts the fraction to milliseconds and raises when the date cannot be encoded.

Edge cases and quirks

  • Fractional input is reduced to millisecond resolution; finer precision is rounded by the Julian conversion chain.
  • The supported result is limited by Delphi's encodable year range, 1 through 9999. A mathematically valid Julian number outside that range still fails.
  • Large Double values lose fractional precision before conversion.
  • The Free Pascal reference is not behaviourally interchangeable: its published documentation states that its corresponding routine is not implemented and raises. Current Velox uses the installed Delphi implementation, which is implemented.
  • The returned TDateTime is untagged; callers must carry the intended astronomical or UTC convention separately.

Side effects

None.

Performance and concurrency

Constant-time floating-point and calendar arithmetic with no external I/O or shared mutable state.

Related entries

  • JulianDateToDateTime accepts the full Julian date number with a noon integer boundary.
  • DateTimeToModifiedJulianDate performs the inverse conversion.

External references

Created 2026-07-15