Thanks for the reply.
So, yes, the number is integer timestamp since the Epoch (e.g. 1422982952874000). Looking over things today, it appears that only JsonView was showing the E+ notation. However, json-cpp is still appending decimal points to my timestamps.
The built-in writers for Json-cpp can optionally take stream, though I see this behavior with and without, and following your advice about the precision.
When I get a chance, I'm going to try a few things and update this thread.
UPDATE: I have updated the thread title to reflect the fact the the problem has to do with the trailing decimal points.