I was about to rant hey that does not even work, it spits out a ton of warnings, how dare you post such untested code, etc etc.. Then I saw I had an old version of Date::Manip. After upgrading from 5.42 to 5.48 it works flawlessly.

# Date::Manip 5.42
]$ /usr/bin/perl -w 679308.pl
Argument "approx" isn't numeric in numeric gt (>) at /usr/lib/perl5/ve
+ndor_perl/5.8.5/Date/Manip.pm line 1908, <DATA> line 24.
Diff between 06/30/96 and 06/30/96 is 0.000000
Argument "approx" isn't numeric in numeric gt (>) at /usr/lib/perl5/ve
+ndor_perl/5.8.5/Date/Manip.pm line 1908, <DATA> line 24.
Diff between 06/30/96 and 08/07/97 is 0.000000
Argument "approx" isn't numeric in numeric gt (>) at /usr/lib/perl5/ve
+ndor_perl/5.8.5/Date/Manip.pm line 1908, <DATA> line 24.
Diff between 06/30/96 and 09/05/97 is 0.000000
[download]

# Date::Manip 5.48
$ /usr/bin/perl 679308.pl
Diff between 06/30/96 and 06/30/96 is 0.000000
Diff between 06/30/96 and 08/07/97 is 13.240246
Diff between 06/30/96 and 09/05/97 is 14.193018
[download]

Owners of older versions of Date::Manip are hereby warned.