I would like to have support for 'date +%s.%N' to get nanoseconds...
nanoseconds is not really what I'm after, but I do like to have milliseconds so I can use it in scripts like this:

start=`date +%s.%N`
sleep 1
echo "`date +%s.%N` - ${start}" | bc
echo "`date +%s.%N` - ${start}" | bc | awk '{printf "%.2f\n", $0}' 

I'm using busybox 1.13 on DD-WRT (standard shell there).