What are the date constraints for SimilarWeb visits API? Python request to traffic API gives 'Dates not in range' error

I'm trying to get data from the SimilarWeb API, using python requests, like this:

import requests
MY_API_KEY = 'XXXXXXXXXXXXXXXXXX'

API_URL = "https://api.similarweb.com/v1/website/{site}/" \
          "total-traffic-and-engagement/visits?api_key={api_key}" \
          "&start_date={start_date}" \
          "&end_date={end_date}" \
          "&main_domain_only=false" \
          "&granularity={granularity}".format(
    site='cnn.com',
    api_key=MY_API_KEY,
    start_date="2017-09",
    end_date="2017-10",
    granularity="monthly"
)

response = requests.get(API_URL)
print response.json()

As far as possible, this follows the similarweb example on their own site here. Only the dates are different.

However, I get a json error stating that my 'Dates not in range' within the json:

{
    u'meta': {
        u'status': u'Error', 
        u'error_code': 101, 
        u'error_message': u'Dates not in range', <--- error msg
        u'request': {
            u'domain': u'cnn.com', 
            u'end_date': u'2017-10-31', 
            u'format': None, 
            u'country': u'world', 
            u'main_domain_only': False, 
            u'limit': None, 
            u'granularity': u'Monthly', 
            u'start_date': u'2017-09-01'
        }
    }
}

I have tried many different dates (including the ones in the example - start_date=2016-01&end_date=2016-03), and I have tried different date formats, but I always get this error. I have tried dates further in the past, and I have tried dates further apart.

If I leave the dates out, I am returned valid data, so I believe the rest of the request must be well formed? I will continue to try different options, but would be really grateful if anyone has had experience with this, as it is driving me a little crazy now!

I am using python 2.7 due to project constraints, with requests version 2.18.4, which I believe to be the most recent available to pip.

You're requesting 2017-09 to 2017-10, Today's Nov 1st so the 2017-10 snapshot is not up yet. contact SimilarWeb support for updates.