How to find mongo document by ObjectID age
The general approach was to fabricate an ObjectID with the time stamp prefix of what I wanted.  The general query looked like

db.mycol.find({_id: {$gte:ObjectId("50b7feb00000000000000000")} }, {fieldIWant:1})


The corresponding mongoexport cmd for a json output file was

 mongoexport -d mydb -c mycol -q '{_id: {$gte:ObjectId("50b7feb00000000000000000")} }' -f 'fieldIWant' &gt; myFile.json