[fpc-pascal] convert "epoch" to UTC

[Sven Barth]

On 16.01.2012 20:06, waldo kitty wrote:
> with this TLE epoch number, 12013.93338171, ya feed it like so...
>
> var
> JEpoch : double;
> DT : TDateTime;
>
> [...]
> JEpoch := getJulianDay_SatEpoch(12,013.93338171);
> [...]
> if TryJulianDateToDateTime(JEpoch,DT) then
> writeln(FormatDateTime('YYYY MM DD hh:mm:ss',DT));
>
>
> the above /should/ return 2012 01 14 22:24:04... i'm getting the right
> dates back but all of the times are 00:00:00 :(
>
> do JulianDateToDateTime and TryJulianDateToDateTime not do anything with
> hours, minutes, seconds and milliseconds??

A quick look at the code reveiled that it doesn't handle it. But at 
least that one should be the same as the one TDateTime uses, so you 
might try the following (which you can then pack into a function yourself):

var
   JEpoch: Double;
   DT: TDateTime;
begin
   JEpoch := getJulianDay_SatEpoch(12,013.9338171);
   if TryJulianDateToDateTime(JEpoch, DT) then begin
     DT := DT + Frac(JEpoch); // the time part should be the one after 
the comma in both Julian and DateTime models
     Writeln(FormatDateTime('YYYY MM DD hh:nn:ss', DT)); // you need to 
use "nn" for minutes as listed here: 
http://www.freepascal.org/docs-html/rtl/sysutils/formatchars.html
   end;
end;

>
> i'd give TryModifiedJulianDateToDateTime a try but the docs state that
> just trying to use that routine raises an exception... is it going to
> handle the hors, minutes, seconds, and milliseconds when it gets done or
> is it for some other julian date format??

The three "Modified Julian Date" functions are implemented as follows, 
so there should be no exception if you use the 
TryModifiedJulianDateToDateTime function (this seems to be another 
documentation error then):

Function DateTimeToModifiedJulianDate(const AValue: TDateTime): Double;
begin
   result := DateTimeToJulianDate(AValue) - 2400000.5;
end;


Function ModifiedJulianDateToDateTime(const AValue: Double): TDateTime;
begin
   result := JulianDateToDateTime(AValue + 2400000.5);
end;


Function TryModifiedJulianDateToDateTime(const AValue: Double; out 
ADateTime: TDateTime): Boolean;
begin
   Result:=TryJulianDateToDateTime(AValue + 2400000.5, ADateTime);
end;


> [ aside: where/how does one report documentation errors? PadLeft says to
> see also PadLeft instead of PadRight ;) ]

You can report all documentation errors at http://bugs.freepascal.org as 
well. Just select as Category "Documentation".

Regards,
Sven