A Julian date (or Julian Day) is a number that refers to the number of days, including fractional days, that have elapsed since a particular historical point in time. The current Julian cycle started at Greenwich noon on January 1, 4713 B.C. (Gregorian calendar), and will end on January 22, 3268 A.D.

Julian date is commonly used by astronomers, geophysicists, and others, and used in computer science to calculate the difference between days, since all numbers in the system are consecutive integers.

For example, the Julian date for December 11, 1942 is 2430705; while December 12, 1942 is 2430706.

This function TryJulianDateToDateTime attempts to convert a Julian Date into its corresponding TDateTime value, and tests whether a valid TDateTime value would be generated.

TryJulianDateToDateTime returns True if it can successfully convert AValue to a TDateTime value, and returns False if it cannot convert AValue into a TDateTime value.

Declaration: function TryJulianDateToDateTime(const AValue: Double; out ADateTime: TDateTime): Boolean;

Here is an easy example, with one intentional failed result.

procedure OnMapEvent(var Value:Variant);
var
TheDate : TDateTime;
TheJDate : double;
aGo : boolean;
begin
//Returns 'The Julian 2460284.5 is 06-12-2023 00:00:00.000'
TheJDate := 2460284.5;
aGo := TryJulianDateToDateTime(TheJDate, TheDate);
If aGo = True then
LogInfo('The Julian '+FloatToStr(TheJDate)+' is '+FormatDateTime('dd-mm-yyyy hh:nn:ss.zzz', TheDate))
else
LogInfo ('Sorry - not a valid attempt.');
//Returns 'Sorry - not a valid attempt.'
TheJDate := 246;
aGo := TryJulianDateToDateTime(TheJDate, TheDate);
If aGo = True then
LogInfo('The Julian '+FloatToStr(TheJDate)+' is '+FormatDateTime('dd-mm-yyyy hh:nn:ss.zzz', TheDate))
else
LogInfo ('Sorry - not a valid attempt.');
//Returns 'The Julian 2430706.59 is 13-12-1942 02:09:36.000'
TheJDate := 2430706.59;
aGo := TryJulianDateToDateTime(TheJDate, TheDate);
If aGo = True then
LogInfo('The Julian '+FloatToStr(TheJDate)+' is '+FormatDateTime('dd-mm-yyyy hh:nn:ss.zzz', TheDate))
else
LogInfo ('Sorry - not a valid attempt.');
end;