Convert any Date format in unix

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http://unix.stackexchange.com – How can I convert any user inputted date into yyyy/mm/dd? For example user can input date in one of the following formats 20120121 , 2012-01-21, 01/21/2012, 01/21/2012 etc. But I need to convert any of the date entered by user into yyyy/mm/dd (2012/01/2012). How? This is the script I was using , but it is not working. echo "Please enter the date: " read X a=$X+"%y/%m/%d" echo $a (HowTos)