#!/bin/sh
EPOCH='jan 1 1970'
sum=''
for i in 00:03:34 00:00:35 00:12:34
do
sum="$(date -u -d "$EPOCH $i" +%s) + $sum"
done
sum="$sum 0"
echo $sum|bc
date -u -d "jan 1 1970" +%s gives 0. So date -u -d "jan 1 1970 00:03:34" +%s gives 214 secs.