import datetime
import json


class DatetimeEncoder(json.JSONEncoder):
    def default(self, obj):
        if isinstance(obj, (datetime.date, datetime.datetime)):
            return obj.isoformat()


def print_and_return(obj):
    for k, v in obj.items():
        if not isinstance(v, str):
            continue
        try:
            dt = datetime.datetime.fromisoformat(v)
        except Exception:
            pass
        try:
            dt = datetime.date.fromisoformat(v)
        except Exception:
            pass
        obj[k] = dt
    return obj


d = dict(user=dict(joined=datetime.datetime.now()), other_user=datetime.date.today())

s = json.dumps(d, cls=DatetimeEncoder)
print(s)
print()

print(json.loads(s, object_hook=print_and_return))