How to pass variable while filtering Mongodb INPUT

{ created_at : { $gte : { $date : "2014-12-31T00:00:00.000Z" } } }

Instead of "2014-12-31T00:00:00.000Z" -> today's date in same format "2014-12-31T00:00:00.000Z"

How can I replace the value with todays date via variable

SOMETHING LIKE BELOW
{ created_at : { $gte : { $date : "$TODAY-DATE" } } }