Depending on how you call Date, the type changes.

wat?

parts = [1982, 5, 29]
// => [1982, 5, 29]
// This is my birthday.

new Date(parts)
// => Sat May 29 1982 00:00:00 GMT+0100 (IST)
// Month is 1-based (5 = May)

new Date(parts[0], parts[1], parts[2])
// => Tue Jun 29 1982 00:00:00 GMT+0100 (IST)
// Month is 0-based (5 = June)

// But what is the Date function?
Date === Date.prototype.constructor
// => true

d = new Date(parts)
// => Sat May 29 1982 00:00:00 GMT+0100 (IST)

s = Date.prototype.constructor.apply({}, parts)
// => "Mon Nov 11 2013 16:05:30 GMT+0000 (GMT)"
// Different answer by calling the function in this manner.

d.constructor
// => function Date() { [native code] }

s.constructor
// => function String() { [native code] }