IIRC, Excel's dates are offset at 1900. So, the following should get you close to a solution:

use Time::JulianDay;

my $jd = 39749 + julian_day(1900, 1, 0);

my ($year, $month, $day) = inverse_julian_day($jd);

print "$year-$month-$day\n";  # 2008-10-29
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(someone else will probably be able to explain why this produces Oct 29, not Oct 28... :)