formatting date with strict json

```python
#!/usr/bin/python

import pandas as pd
import json

master_df = pd.read_csv('sg_main__test.txt', parse_dates={'$date' :['date', 'time']}, sep='\s+', encoding='utf-8')

out_file = master_df.to_json('sg_main__test.json', date_format='iso', orient='records')
```
#### Problem description

I need my data set to export to - "Timestamp" : {$date:"2018-02-12T01:50:37.000Z"} Currently its exporting as - {"$date":"2011-07-22T20:34:51.000Z"

From my digging this isn't currently possible?