date from string and format missing year is valid

When making a date from string and format, I expect a missing year to make the date invalid.
http://jsfiddle.net/jw8o2g2a/2/

674: The toInt function should not default to 0 when the argument is undefined.  This returns a technically valid year even though no year was provided.

2059: parseTwoDigitYear attempts to take an input value and add 1900 or 2000 to it.  If the value is undefined, this will return NaN, so default to 0.

1361: use DATE constant rather than hard coded value
Please revert this change. Its not related to the `toInt` problem.