Different date formats in Unix

Date Format: 

1 > $a=`date '+%d%m%y_%H%M%S'`

$echo $a

130812_195306

2 >$a=`date '+%d%m%Y_%H%M%S'`

$echo $a

13082012_195409

3 >$a `date '+%d-%m-%Y %H:%M:%S'`

$echo $a

13-08-2012 19:57:13

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