core-js/modules/es6.date.to-string.js

var DateProto = Date.prototype;
var INVALID_DATE = 'Invalid Date';
var TO_STRING = 'toString';
var $toString = DateProto[TO_STRING];
var getTime = DateProto.getTime;
if (new Date(NaN) + '' != INVALID_DATE) {
  require('./_redefine')(DateProto, TO_STRING, function toString() {
    var value = getTime.call(this);
    // eslint-disable-next-line no-self-compare
    return value === value ? $toString.call(this) : INVALID_DATE;
  });
}