Perl sub converts a Julian date number (JDN) into its Gregorian year, month, day, components not 100% precise for JDN too far in the past, but seems good for modern dates (algorithm & constants from Wikipedia)

julian2ymd.pl

# convert julian date number into Gregorian year, month, day parts
# not precise too far in the past... 1580-something?
sub jdn2greg {
  my ($jdate) = @_;

	# make sure formatted like JDN (DDDDDDD[.D*])
	return if $jdate !~ m/^\d{7}\.?(\d*)$/;
  # TODO add support for time here based on fractional day RH side of decimal place
	# define conversion constants
	my $y = 4716;
	my $v = 3;
	my $j = 1401;
	my $m = 2;
	my $n = 12;
	my $u = 5;
	my $s = 153;
	my $w = 2;
	my $r = 4;
	my $B = 274277;
	my $p = 1461;
	my $C = -38;

  # do some math
	my $f = $jdate + $j;
	$f = $f + $C + int( ( int( ( 4 * $jdate + $B ) / 146097 ) * 3 ) / 4 );
	my $e     = $r * $f + $v;
	my $g     = int( ( $e % $p ) / $r );
	my $h     = $u * $g + $w;
	my $day   = int( ( $h % $s ) / $u ) + 1;
	my $month = ( ( int( $h / $s ) + $m ) % $n ) + 1;
	my $year
		= int( $e / $p ) - $y + int( ( $n + $m - $month ) / $n );

	return ($year, $month, $day);
}