The computation used by tv_grab_na_dd for --offset is incorrect

Forwarded from https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=814819

Package: xmltv-util
Version: 0.5.63-2
Severity: normal

tv_grab_na_dd uses the --offset option in a way that does not allow negative
offsets (ie. grabbing data for the previous X days). The attached patch
fixes the issue, and does not break positive offsets.

-- System Information:
Debian Release: 8.3
APT prefers stable-updates
APT policy: (500, 'stable-updates'), (103, 'stable')
Architecture: amd64 (x86_64)
Foreign Architectures: i386

Kernel: Linux 4.3.0-0.bpo.1-amd64 (SMP w/4 CPU cores)
Locale: LANG=en_CA.utf8, LC_CTYPE=en_CA.utf8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash
Init: sysvinit (via /sbin/init)

Versions of packages xmltv-util depends on:
pn libarchive-zip-perl <none>
ii libdata-dump-perl 1.22-1
ii libdate-manip-perl 6.47-1
pn libdatetime-format-iso8601-perl <none>
pn libdatetime-format-strptime-perl <none>
ii libdatetime-perl 2:1.12-1
ii libdatetime-timezone-perl 1:1.75-2+2015g
ii libhtml-parser-perl 3.71-1+b3
pn libhtml-tableextract-perl <none>
ii libhtml-tree-perl 5.03-1
pn libhttp-cache-transparent-perl <none>
ii libio-stringy-perl 2.110-5
ii libjson-perl 2.61-1
ii libparse-recdescent-perl 1.967009+dfsg-1
pn libsoap-lite-perl <none>
ii libterm-readkey-perl 2.32-1+b1
pn libtext-bidi-perl <none>
ii libtext-iconv-perl 1.7-5+b2
pn libwww-mechanize-perl <none>
ii libwww-perl 6.08-1
pn libxml-dom-perl <none>
pn libxml-libxml-perl <none>
pn libxml-libxslt-perl <none>
ii libxml-parser-perl 2.41-3
ii libxml-twig-perl 1:3.48-1
pn libxml-writer-perl <none>
pn libxmltv-perl <none>
ii perl [libio-compress-perl] 5.20.2-3+deb8u3
ii perl-modules 5.20.2-3+deb8u3</none></none></none></none></none></none></none></none></none></none></none></none></none>

Versions of packages xmltv-util recommends:
pn liblingua-preferred-perl <none>
pn libterm-progressbar-perl <none>
ii libunicode-string-perl 2.09-5+b2</none></none>

Versions of packages xmltv-util suggests:
pn liblinux-dvb-perl <none>
pn liblog-tracemessages-perl <none>
pn libtext-kakasi-perl <none>
pn xmltv-gui <none></none></none></none></none>