Backup with mongoexport or mongodump?
(sorry, dunno)
--query will select or exclude entire documents, not fields. 


It would be a good idea though :  

--query '{datetime:{$gt:ISODate("2014-01-01T00:00:00.000Z")}},{_id:0,name:1,address:1,interests:1}'* )



Mongodump uses bson file structure and preserves the data types. Mongoexport will lose data type of the values. Such as NumberLong("1431677405876") would be converted to just 1431677405876. That's why mongodump is advised if you need to import back to MongoDB.