Correct error message for date format

The date format is actually "YYYY-MM-DD", not "YY-MM-DD".
Correct date format in json data request

It was locale dependent, but should not be.
Now getting the correct format YYYY-MM-DD as the first 10 characters of
the ISO datetime string.
Merge pull request #70 from joelgoop/fix-parliament-figures

Fix an error in formatting of date strings