Have you tried typing "date arithmetic" into the search box? Or looking into the "dates and times" section of "Q&A"? That should help...

update: (noting with some personal remorse that this was your first post...) Sorry to be less than helpful. Welcome! The short answer would be something like:

use Date::Manip;

$currentDate = UnixDate( "today", "%Y_%m_%d" );
$currentMinusThree = UnixDate( "3 days ago", "%Y/%m/%d" );
print join " ", $currentDate, $currentMinusThree, "\n";
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In other words, don't bother with trying to munge the output of the shell "date" command -- perl modules make things a lot easier than that.