DateTimeToModifiedJulianDate

Function DateTimeToModifiedJulianDate(const AValue: TDateTime): Double

Example

procedure ScriptEvent(var Value: variant);
begin
  Value := DateTimeToModifiedJulianDate(EncodeDate(2000, 1, 1));
  // Result: 51544.0
end;

Usage

DateTimeToModifiedJulianDate converts a TDateTime value to a Modified Julian date by applying the 2400000.5 offset.

Parameters

Name	Type	Description
AValue	TDateTime, const	Gregorian date/time to convert. No timezone is attached or inferred.

Returns

A Double equal to DateTimeToJulianDate(AValue) - 2400000.5. MJD 0.0 corresponds to midnight at the start of 17 November 1858.

Behaviour

Civil midnight produces an integral Modified Julian value; later times appear as the fractional part of that day. The operation is purely numeric and leaves the input's local/UTC interpretation to the caller.

Errors

Valid Velox dates do not raise. Conversion errors are raised unchanged to the script.

Additional Technical Info

DateTimeToModifiedJulianDate returns the Modified Julian Date (MJD) for a TDateTime. MJD is the ordinary Julian date minus 2400000.5, which moves the day boundary from noon to midnight and produces smaller day numbers.

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

Implementation

The DateUtils scripting import binds directly to Delphi 37.0 System.DateUtils.DateTimeToModifiedJulianDate. The function calls the same unit's DateTimeToJulianDate and subtracts its CJDToMJDOffset constant (2400000.5). All Gregorian decoding and time-fraction behaviour therefore comes from that helper.

Edge cases and quirks

• Every Julian conversion caveat applies: no timezone conversion, floating precision limits, Abs(Frac(AValue)) for negative Delphi dates and no local input validation.
• MJD is not a Unix timestamp. Its unit is a day and its epoch is in 1858, not 1970.
• Values derived by floating arithmetic may need tolerance-based comparison.
• Free Pascal documents the same MJD = JD - 2400000.5 convention. It remains a dialect reference; the terminal used by Velox is Delphi.

Side effects

None.

Performance and concurrency

Constant-time scalar arithmetic with no allocation or shared state.

Related entries

• DateTimeToJulianDate returns the unmodified noon-boundary value.
• DateTimeToUnix returns seconds from the 1970 epoch instead of fractional days.

External references

• Embarcadero System.DateUtils.DateTimeToModifiedJulianDate - documents the exact Delphi function.
• Free Pascal DateTimeToModifiedJulianDate - explains the compatible Modified Julian convention and offset.

Created 2026-07-15