to_json() to_csv() don't act the same for dates in iso format
I posted it on stackoverflow but this seems more like a bug / missing feature?
https://stackoverflow.com/questions/53494416/pandas-data-frame-to-json-to-csv-dont-act-the-same-for-dates-in-iso-forma


In Json the dates appear as:  `"lastDeliveryDate":1543258826689`

In CSV the dates appear as:
```
lastDeliveryDate
2018-11-21 16:25:53.990000-05:00
```

Why the Json show it as unreadable number? 

I should note this doesn't happen for "regular" formats of dates.
This happens in dates as iso_format 

What can I do to see the dates in Json as I see them in the CSV?