Trying to add 3 days in Milliseconds to current Date

To add time, get the current date then add, as milliseconds, the specific amount of time, then create a new date with the value:

// get the current date & time (as milliseconds since Epoch)
const currentTimeAsMs = Date.now();

// Add 3 days to the current date & time
//   I'd suggest using the calculated static value instead of doing inline math
//   I did it this way to simply show where the number came from
const adjustedTimeAsMs = currentTimeAsMs + (1000 * 60 * 60 * 24 * 3);

// create a new Date object, using the adjusted time
const adjustedDateObj = new Date(adjustedTimeAsMs);

To explain this further; the reason dataObj.setMilliseconds() doesn’t work is because it sets the dateobj’s milliseconds PROPERTY to the specified value(a value between 0 and 999). It does not set, as milliseconds, the date of the object.

// assume this returns a date where milliseconds is 0
dateObj = new Date();

dateObj.setMilliseconds(5);
console.log(dateObj.getMilliseconds()); // 5

// due to the set value being over 999, the engine assumes 0
dateObj.setMilliseconds(5000);
console.log(dateObj.getMilliseconds()); // 0

References:
Date.now()
new Date()
Date.setMilliseconds()