Convert Delphi Formatted Date Time to regular DateTime via PHP

Hi,

I am converting a script written in ASP to PHP. It reads a file created in Delphi. One particular function converts the Delphi-formatted times (e.g. 38837.0129173727) to a string of normal date time format (DD/MM/YYYY HH:MM:SS).

Here is the script

function ConvertDateTime(DelphiDateTime,DateTime)
      Days = Int(DelphiDateTime)
       Fraction = DelphiDateTime - Days
      Seconds = Fraction / (1/24/60/60)
      DateTime = CStr(DateAdd("d",Days,"30/12/1899 00:00:00"))
      DateTime = CStr(DateAdd("s",Seconds,DateTime))      
End Function

I am trying to convert this to PHP but really have no idea how delphi times are formatted in the first place. This function seems odd because the two arguments are exactly the same and are both the same delphi datetime values to be converted.

I've converted to PHP as

function ConvertDateTime($DelphiTime,$DateTime) {
      $Days = intval($DelphiTime);
      $Fraction = $DelphiTime - $Days;
      $Seconds = $Fraction / (1/24/60/60);
      $DateTime = strval(DateAdd("d",$Days,"30/12/1899 00:00:00"));
      $DateTime = strval(DateAdd("s",$Seconds,$DateTime));
      return $DateTime;
}

But when I pass echo ConvertDateTime(38837.0129173727,38837.0129173727) I get

Warning: mktime(): Windows does not support negative values for this function in ...

I am using a standard DateAdd function in PHP

function DateAdd($interval, $number, $date) {

    $date_time_array = getdate($date);
    $hours = $date_time_array['hours'];
    $minutes = $date_time_array['minutes'];
    $seconds = $date_time_array['seconds'];
    $month = $date_time_array['mon'];
    $day = $date_time_array['mday'];
    $year = $date_time_array['year'];

    switch ($interval) {
   
        case 'yyyy':
            $year+=$number;
            break;
        case 'q':
            $year+=($number*3);
            break;
        case 'm':
            $month+=$number;
            break;
        case 'y':
        case 'd':
        case 'w':
            $day+=$number;
            break;
        case 'ww':
            $day+=($number*7);
            break;
        case 'h':
            $hours+=$number;
            break;
        case 'n':
            $minutes+=$number;
            break;
        case 's':
            $seconds+=$number;
            break;            
    }
       $timestamp= mktime($hours,$minutes,$seconds,$month,$day,$year);
    return $timestamp;
}

How are these delphi times actually formatted and how might I use this knowledge to develop a script to convert them using PHP?