var objIdMin = ObjectId(Math.floor((new Date('1990/10/10'))/1000).toString(16) + "000
0000000000000")

var objIdMax = ObjectId(Math.floor((new Date('2011/10/10'))/1000).toString(16) + "000
    0000000000000")

db.myCollection.find({_id:{$gt: objIdMin, $lt: objIdMax}})