How to hack the Date object to return a shifted time

Question

I'm trying to test an extension designed to work only in my workplace (intranet). More specifically, I placed breakpoints in points of an alarm-triggered routine, where I read the system time at 2 specific times of the day; one close to midnight and the other around 05:00 P.M. In order not to have to wait for these two moments of the day, I would like to move the system time close to these two times, but I can't do it because system administrators have disabled this option.

So I'm here to ask for your help. It is possible to "override" the Date object so that it returns a fake time shifted (forward or backward) by a constant value decided by me on the spot.

Furthermore, time must flow normally.

Taking a practical example, if I now launch this command in the console:
dt = new Date()

I get

Fri Apr 14 2023 10:50:00 GMT+0200

after overriding it should display

Fri Apr 14 2023 23:50:00 GMT+0200

I want the time moved forward 13 hours. Of course I should activate this movement with a command like:

shiftDate.move(13)

and to make things right again something like:

shiftDate.reset();

All methods\properties of the Date object must remain unchanged.

Can you give me a hand please?

Thanks in advance

Answer 1

This article might help. https://www.browserstack.com/guide/change-time-zone-in-chrome-for-testing

Try Method 1: Using Developer Tools to Change Chrome Timezone in particular.

1. Open DevTools in Chrome -> Open the Console drawer.
2. Click on the three-dotted menu -> Click on More tools -> Sensors.
3. From the Sensors tab, set the location according to your preference and define the specific timezone.

Answer 2

I found a solution, perhaps trivial but it seems to work.

const fakeDate = {
    set:        function(y, m, d, h, m, s) {
                    if (typeof DateObjBck !== 'undefined')
                        this.reset();

                    /*Saving the original Date object */
                    DateObjBck = Date;

                    /* set the clock closed to midnight */
                    var startFrom = new Date(...arguments);

                    /*Calculating the constat offset between real time and the fake one */
                    const offsetMs = startFrom.getTime() - (new DateObjBck).getTime();
                    
                    /* Now I rewrite the Date object */
                    Date =  function(...args) {
                                if (args.length > 0)
                                    return new DateObjBck(...args);
                                else {
                                    var veryNow = (new DateObjBck).getTime();
                                    return new DateObjBck(veryNow + offsetMs)                                   
                                }
                            };
                    Date.now =  function() {
                                    var veryNow = (new DateObjBck).getTime();
                                    return veryNow + offsetMs
                                }
                },
    reset:  function() {
                    Date = DateObjBck;
                    DateObjBck = undefined
            }
}

fakeDate.set(2023, 3, 13, 23, 55, 0);
console.log('0', new Date());
console.log('1', new Date);
console.log('2', Date.now());
console.log('3', new Date(2020, 5, 15, 22, 50, 0));
console.log('4', new Date);
fakeDate.reset();
console.log('5', new Date);

Answer 3

Because Date is a class, we can extend it to provide additional functionality without tampering with the existing static properties/methods.

For instance, you can write a new class like ShiftedDate below:

class ShiftedDate extends Date{
    static #offsetMs = 0; //private static field
    static move(ms){
        this.#offsetMs = ms;
    }
    static moveHours(hours){
        this.move(hours*60*60*1000); 
    }
    static setTime(...args){
        this.move(new Date(...args)-new Date());
    }
    static now(){ //The only static function I can think of that doesn't depend on instance time. 
        return super.now()+this.#offsetMs;
    }
    constructor(...args){
        super(...args); 
        if(args.length===0)this.setTime(this.getTime()+ShiftedDate.#offsetMs); //Don't shift explicit dates
    }
}

And use it as such:

const originalDate = new ShiftedDate();
console.log(new ShiftedDate()); //2023-04-15T11:49:01.943Z

ShiftedDate.moveHours(2);
console.log(new ShiftedDate()); //2023-04-15T13:49:02.018Z

ShiftedDate.setTime("7/12/2012");
console.log(new ShiftedDate()); //2012-07-12T00:00:00.000Z

console.log(originalDate); //2023-04-15T11:49:01.943Z - Old dates are not updated
console.log(ShiftedDate.now()); //1342051200000 (7/12/12)