US woman drinks petrol

Washington: Usually people around the world like to drink different juices but there is a US woman who is addicted to drink petrol.
Shannon, set to appear on the aptly named TV show My Strange Addiction, claims to drink up to 12 teaspoons of petrol per day.
“It tingles at first and then it burns the back of my throat,” the US woman told TV channel TLC. “Even though it hurts me, it makes me feel good.”
According to TLC, Shannon whose preferred method of drinking petrol is straight out of the canister, has consumed over 18 litres in the last year.
“I take the cap off and breathe in the fumes that come forcing out of the can,” she said.
Shannon’s mother said at first she didn’t believe her daughter was addicted to petrol, but then she smelled it in her breath.

Elapsed time in UNIX shell

The other day I had enough calculating the time so I just wrote this script:
cat > ~/bin/timediff
#!/bin/sh
if [ -z $2 ]; then
echo "usage: $0 "
echo
echo "ex. $0 11:49 12:51"
exit 1
fi

end=`date +%s -d"$2"`
start=`date +%s -d"$1"`

diff=$(($end-$start))

diff=$(($diff/60))
hours=$(($diff/60))
min=$(($diff%60))
# Prepend a 0 if minute <10
if [ ${#min} -eq 1 ]; then
min=0$min
fi

echo "$hours:$min"

Of course, it works in some cases only. This one is fine:
$ sh ~/bin/timediff 11:49 12:51
1:02

Whereas this one will not work:
$ sh ~/bin/timediff 11:49 1:51
-9:-58

But the output is quite obvious.