Removed obsolete week offset calculation from week picker's day changing code.
YYYY-MM-DD strings are formatted into ISO 8601 representation before they are used creating a Date object.
This fixes at least the day slide bug in timezones behind UTC, and most likely other incorrect dates elsewhere. See https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date/parse\#ECMAScript_5_ISO-8601_format_support and http://en.wikipedia.org/wiki/ISO_8601 for additional information.