Вы ищете date_format='iso'
в df.to_json()
Полный пример:
import pandas as pd
csvdata = '''\
date
2018-05-01
2018-05-02
2018-05-03'''
fileobj = pd.compat.StringIO(csvdata)
df = pd.read_csv(fileobj, sep='\s+')
# without conversion
print(df.to_json())
# with conversion
df['date'] = pd.to_datetime(df['date'])
print(df.to_json())
# with conversion and adding date_format = iso
print(df.to_json(date_format='iso'))
Отпечатки:
{"date":{"0":"2018-05-01","1":"2018-05-02","2":"2018-05-03"}}
{"date":{"0":1525132800000,"1":1525219200000,"2":1525305600000}}
{"date":{"0":"2018-05-01T00:00:00.000Z","1":"2018-05-02T00:00:00.000Z","2":"2018-05-03T00:00:00.000Z"}}