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export _GNUDATE="date"
if [ "${OSTYPE:0:6}" = "darwin" ] && command -v gdate >/dev/null 2>/dev/null ; then
  export _GNUDATE="gdate"
fi

date2stamp() {
  $_GNUDATE --utc --date "$1" +%s
}
stamp2date() {
  $_GNUDATE --utc --date "1970-01-01 $1 sec" "+%Y-%m-%d %T"
}
dateDiff() {
  case $1 in
    -s)   sec=1;      shift;;
    -m)   sec=60;     shift;;
    -h)   sec=3600;   shift;;
    -d)   sec=86400;  shift;;
    *)    sec=86400;;
  esac
  dte1=$(date2stamp $1)
  dte2=$(date2stamp $2)
  diffSec=$((dte2-dte1))
  if ((diffSec < 0)); then abs=-1; else abs=1; fi
  echo $((diffSec/sec*abs))
}
date_range() {
  range=$(dateDiff -d $1 $2)
  for((i=0; i<=$range; i++)) ; do
    $_GNUDATE --date "$1 $i day" +'%Y-%m-%d'
  done
}