how to search mongodb fieldtype timestamp?(symfony2)

I use Mongodb.

and create query.

createAt field type is timestamp.
ex) createAt => new MongoTimestamp(1361925805, 0),

My query is
ex)
...
->field('createAt')->lt(1361925805)
...

but result is array(0).
ex) select * from aaa where createAt < '125123123123'

how to make mongodb query.