[{state:"California",date:"2010-08-11",otherData:"Something Else"},{state:"California",date:"2010-08-12",otherData:"Something Else"},{state:"New York",date:"2010-02-09",otherData:"Something Else"},{state:"New York",date:"2010-02-11",otherData:"Something Else"}]

db.collection.aggregate([{$project:{state:1,date:{$dateFromString:{dateString:"$date",format:"%Y-%m-%d"}}}},{$group:{_id:"$state",mostRecentDate:{$max:"$date"}}},{$project:{state:"$_id",date:{$dateToString:{date:"$mostRecentDate",format:"%Y-%m-%d"}},_id:0}}])