Javascript: comparing two values always not equal

ECMAScript 2018 Standard Method

You would use object spread:

let merged = {...obj1, ...obj2};

merged is now the union of obj1 and obj2. Properties in obj2 will overwrite those in obj1.

/** There's no limit to the number of objects you can merge.
 *  Later properties overwrite earlier properties with the same name. */
const allRules = {...obj1, ...obj2, ...obj3};

Here is also the MDN documentation for this syntax. If you're using babel you'll need the babel-plugin-transform-object-rest-spread plugin for it to work.

ECMAScript 2015 (ES6) Standard Method

/* For the case in question, you would do: */
Object.assign(obj1, obj2);

/** There's no limit to the number of objects you can merge.
 *  All objects get merged into the first object. 
 *  Only the object in the first argument is mutated and returned.
 *  Later properties overwrite earlier properties with the same name. */
const allRules = Object.assign({}, obj1, obj2, obj3, etc);

(see MDN JavaScript Reference)

Method for ES5 and Earlier

for (var attrname in obj2) { obj1[attrname] = obj2[attrname]; }

Note that this will simply add all attributes of obj2 to obj1 which might not be what you want if you still want to use the unmodified obj1.

If you're using a framework that craps all over your prototypes then you have to get fancier with checks like hasOwnProperty, but that code will work for 99% of cases.

Example function:

/**
 * Overwrites obj1's values with obj2's and adds obj2's if non existent in obj1
 * @param obj1
 * @param obj2
 * @returns obj3 a new object based on obj1 and obj2
 */
function merge_options(obj1,obj2){
    var obj3 = {};
    for (var attrname in obj1) { obj3[attrname] = obj1[attrname]; }
    for (var attrname in obj2) { obj3[attrname] = obj2[attrname]; }
    return obj3;
}

The Date object will do what you want - construct one for each date, then compare them using the >, <, <= or >=.

The ==, !=, ===, and !== operators require you to use date.getTime() as in

var d1 = new Date();
var d2 = new Date(d1);
var same = d1.getTime() === d2.getTime();
var notSame = d1.getTime() !== d2.getTime();

to be clear just checking for equality directly with the date objects won't work

var d1 = new Date();
var d2 = new Date(d1);

console.log(d1 == d2);   // prints false (wrong!) 
console.log(d1 === d2);  // prints false (wrong!)
console.log(d1 != d2);   // prints true  (wrong!)
console.log(d1 !== d2);  // prints true  (wrong!)
console.log(d1.getTime() === d2.getTime()); // prints true (correct)

I suggest you use drop-downs or some similar constrained form of date entry rather than text boxes, though, lest you find yourself in input validation hell.

For the curious, date.getTime() documentation:

Returns the numeric value of the specified date as the number of milliseconds since January 1, 1970, 00:00:00 UTC. (Negative values are returned for prior times.)