Description:
------------
I'm generating a function to calculate a date in Julian day count to be used in excel spreadsheet. Always I obtain a 2 days difference. Is this a bug?

Reproduce code:
---------------
<?php
// This is the date that I would to convert
$jd1 = GregorianToJD (8,28,2003);
print "$jd1\n";
$gregorian1 = JDToGregorian ($jd1);
print "$gregorian1\n";
// This is the initial excel's date counter
$jd2 = GregorianToJD (1,1,1900);
print "$jd2\n";
$gregorian2 = JDToGregorian ($jd2);
print "$gregorian2\n";
// This is the REAL (not bogus) day counter
$total=$jd1 - $jd2 + 2;
print "Excel value: ".$total;
?>

Expected result:
----------------
Without the correction ($total=$jd1 - $jd2;) the result is:
2452880 8/28/2003
2415021 1/1/1900
Real counter: 37859

Whit the correction ($total=$jd1 - $jd2 + 2;) the result is:
2452880 8/28/2003
2415021 1/1/1900
Real counter: 37861

First one isn't correct, second one is fine.