We have a json encoder which overrides the default encoder so that timestamps are encoded as ISO-8601 strings ...

def our_default_encoder(obj):
    """Override the json encoder that comes with pymongo (bson)
    So that datetime objects are encoded as ISO-8601"""
    if isinstance(obj, datetime.datetime):
        iso = obj.isoformat()
        return {"$date": iso}
    return json_util.default(obj)

Even though the bson decoder in json_util parses some ISO-8601 strings it assumes that the seconds field contains 3 decimal places (i.e. milliseconds)

Could the object hook be augmented to parse a wider range of ISO-8601 timestamps? Possibly by ...

import iso8601

def object_hook(dct):
   ...
    if "$date" in dct:
        dtm = dct["$date"]
        if isinstance(dtm, string_type):
            dt = iso8601.parse_date(dtm)
            return dt
   ...

I have tested this against the current set of unittests and it appears to maintain the current behaviour and will parse strings generated by isoformat()