Fixing working time

#!/bin/bash
echo '--------------------'
echo '   Work Time v1.1   '
echo '--------------------'
SECONDS_FILE='seconds.dat'
TIMES_FILE='times.dat'

convertsecs() {
 ((h=${1}/3600))
 ((m=(${1}%3600)/60))
 ((s=${1}%60))
 printf "%02d:%02d:%02d\n" $h $m $s
}

OPTION=exit
function mainMenu {
    if [ -f "start" ];  then
        OPTION=$(whiptail --title  "Work Time" --menu  "Select operation" 20 60 12 \
        "stop" "Stop" \
        "times" "Time" \
        "clean" "Reset" \
        3>&1 1>&2 2>&3)
    else
        OPTION=$(whiptail --title  "Work Time" --menu  "Select operation" 20 60 12 \
        "start" "Start" \
        "times" "Time" \
        "clean" "Reset" \
        3>&1 1>&2 2>&3)

    fi
}


mainMenu
estatus=$?
if [ $estatus = 1 ];  then
    exit
fi
timestamp=$(date +%s)
curtime=`date +%Y-%m-%d_%H:%M:%S`

if [ $OPTION = "times" ];  then
    echo 'Times'
    cat times.dat
    res=`awk '{ sum += $1 } END { print sum }' $SECONDS_FILE`
    echo $res
    times=$(convertsecs $res)
    echo $times
    echo 'All times '$times > alltime
    whiptail --textbox alltime 12 80
    exit
fi

if [ $OPTION = "clean" ];  then
    zip $timestamp'.zip' $SECONDS_FILE $TIMES_FILE alltime start starttime
    rm -f $SECONDS_FILE $TIMES_FILE alltime start starttime
    echo 'Clean ok.'
    exit
fi

echo $timestamp
echo $curtime


if [ $OPTION = "start" ];  then
    echo 'Start Time'
    echo $timestamp > start
    echo $curtime > starttime
    exit
fi

if [ $OPTION = "stop" ];  then
    timestamp=$(date +%s)
    echo 'Stop Time'
    startstamp=`cat start`
    seconds="$((timestamp-startstamp))"
    starttime=`cat starttime`
    echo $starttime $curtime $seconds >> $TIMES_FILE
    echo $seconds >> $SECONDS_FILE
    rm -f start starttime
    exit
fi