#!/bin/zsh

rDate() { y=`echo ${1} | cut -d- -f1`; m=`echo ${1} | cut -d- -f2`
  d=`echo ${1} | cut -d- -f3`; rday=`echo $d-$m-$y`; return; }

day1="2018-04-17" 
echo $day1
rDate $day1
echo $rday


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> source rDate.sh
2018-04-17
17-04-2018