I'm not sure if this is a display bug or something else.

This works fine:

var doc = db.data.rrd.findOne(

{cid: ObjectId('XXX'), g: 'opcounters', i: 'query', d: '20110203', hid : 'YYY'}

,

{'mn.1330.t': 1}

);
print(doc.mn['1330'].t + 1);

E.g.

"1406" :

{ "n" : 2, "t" : NumberLong("4209660892") }

,
"1407" :

{ "n" : 1, "t" : NumberLong(2104973521) }