[BUG?] pd.read_json does not convert date before 1971-01-01
#### Code Sample

```python
from datetime import datetime
import pandas as pd
input_before_31536000 = pd.DataFrame([1],index=[datetime(1971, 1, 1)])
input_after_31536000 = pd.DataFrame([1],index=[datetime(1971, 1, 1, 0, 0, 1)])

# Compare before (including) and after 31536000
pd.read_json(input_before_31536000.to_json(date_unit='s'), date_unit='s')
pd.read_json(input_after_31536000.to_json(date_unit='s'), date_unit='s')
```

#### Problem description
Read json does not convert epoch timestamps before and including 31536000 into dates. We start with the following input:

```
            0
1971-01-01  1
```
and after a roundtrip encoding/decoding transforms into:
```
          0
31536000  1
```

Any date after `datetime(1971, 1, 1)` get converted correctly.

#### A warkaround
I am using epoch timestamps and `orient='split'` to keep the encoding as terse as possible. ISO dates would solve the issue but would take much more space. To keep epoch timestamp here is a manual workaround:

```python
import json

# Encode series
json_str = input_before_31536000[0].to_json(orient='split', date_unit='s')
data = json.loads(json_str)
# Build DataFrame manually with to_datetime() conversion
pd.DataFrame(data['data'], index=pd.to_datetime(data['index'], unit='s'), columns=[data['name']])
```
#### Output of ``pd.show_versions()``

<details>

```
INSTALLED VERSIONS
------------------
commit: None
python: 3.7.2.final.0
python-bits: 64
OS: Darwin
OS-release: 18.2.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_GB.UTF-8
LOCALE: en_GB.UTF-8

pandas: 0.24.0
pytest: None
pip: 19.0.1
setuptools: 40.7.3
Cython: None
numpy: 1.16.1
scipy: None
pyarrow: None
xarray: None
IPython: 7.2.0
sphinx: None
patsy: None
dateutil: 2.7.5
pytz: 2018.9
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: None
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml.etree: 4.3.0
bs4: None
html5lib: None
sqlalchemy: 1.2.17
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: 0.2.1
pandas_gbq: None
pandas_datareader: 0.7.0
gcsfs: None
```

</details>
