See #9798 (comment)

Can be reproduced with this URL: http://demo.piwik.org/?module=API&method=Live.getLastVisitsDetails&idSite=7&period=day&date=last10&format=json&token_auth=anonymous&translateColumnNames=1

In the example there is period=day&date=last10 and it results in an error

Error: Date format must be: YYYY-MM-DD, or 'today' or 'yesterday' or any keyword supported by the strtotime function (see http://php.net/strtotime for more information):