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TryModifiedJulianDateToDateTime

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

Example​

procedure ScriptEvent(var Value: variant);
var
  EncodedValue: TDateTime;
begin
  if TryModifiedJulianDateToDateTime(51544.0, EncodedValue) then
    Value := EncodedValue; // 1 January 2000 at 00:00
end;

Usage​

TryModifiedJulianDateToDateTime tries to convert a Modified Julian date into an encoded Gregorian date and millisecond time.

Parameters

NameTypeDescription
AValueDouble, constModified Julian date, including an optional fraction since midnight.
ADateTimeTDateTime, varReceives the millisecond-resolution conversion when successful.

Returns

True when the adjusted Julian date can be converted and ADateTime receives the result; otherwise False.

Errors

Unencodable derived dates normally return False; floating-point conversion or arithmetic faults can propagate.

Additional Technical Info​

TryModifiedJulianDateToDateTime converts a Modified Julian date (MJD) to a proleptic-Gregorian TDateTime. MJD uses a midnight day boundary and an epoch at 17 November 1858, making 51544.0 midnight on 1 January 2000.

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.TryModifiedJulianDateToDateTime. Its entire wrapper adds 2,400,000.5 and delegates to TryJulianDateToDateTime. That terminal derives the Gregorian date, accounts for the Julian noon boundary and adds the fractional day at millisecond resolution.

Edge cases and quirks

  • The .5 in the offset is what converts the underlying Julian noon boundary to MJD's midnight boundary.
  • Output is limited to the Delphi calendar range, normally returning False for an unencodable derived date.
  • Adding the large offset can lose low-order fractional precision for extremely precise or large Double inputs before millisecond quantisation occurs.
  • NaN, infinity, extreme numeric input and boundary timestamp arithmetic can raise despite the Try name because the delegated routine does not catch all numeric operations.
  • ADateTime has no timezone or astronomical time-scale tag.

Side effects

May write ADateTime; no external state changes.

Performance and concurrency

Constant-time arithmetic with no allocation, I/O or shared mutable state.

Related entries

External references

Created 2026-07-15