#!/bin/bash

days="90"
now=$(date +%s)

days_in_seconds=$((60*60*24*${days}))
limit=$((${now} - ${days_in_seconds}))

for snap in $(zfs list -r -t snapshot | grep -v NAME | awk '{print $1}'); do

    snap_time=$(echo ${snap} | awk -F@ '{print $2}')

    if [[ ${snap_time} -lt ${limit} ]]; then

        zfs destroy ${snap}

    fi

done