Convert any date format into yyyy/mm/dd

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http://www.unix.com – How can I convert any user inputted date into yyyy/mm/dd ? For example user can input date one of the following 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). Any suggestion. Thanks in advance this is the script I was using echo "Please enter the date: " read X a=$X+"%y/%m/%d" echo $a (HowTos)