Fixing 'date is too far in the past' API error

[arithmetric]

This PR updates the default dates used with the availability API to avoid a 'date is too far in the past' error from the Cronofy API.

Example errors in API response:

{
    "errors": {
        "query_periods[0].start": [
            {
                "key": "errors.min_date_exceeded",
                "description": "date is too far in the past"
            }
        ],
        "query_periods[1].start": [
            {
                "key": "errors.min_date_exceeded",
                "description": "date is too far in the past"
            }
        ]
    }
}