Calculations in shell Unix

I'm trying to work out the difference in 2 unix times to do a simple calculation (in a shell script) but it doesn't seem to work. I have set 2 variables one called $d and the other $c. Here's the syntax i currently have for setting up the variables: c= echo $a | awk -F : '{print ($1 * 32140800) + ($2 * 2678400) + ($3 * 86400) + ($4 * 3600) + ($5 * 60) }' echo $c d= echo $a | awk -F : '{print ($1 * 32140800) + ($2 * 2678400) + ($3 * 86400) + ($4 * 3600) + ($5 * 60) }' echo $d (variables a and b simply receive the timestamp from another script) I'm trying to subtract the output of $c from $d but every method I have used doesn't seem to work. I've tried the following: 1) duration=$(echo "$d - $c") echo $duration 2) duration=$((d-c)) echo $duration