JavaScript function for the get a specifed weekday n-weeks before / in the future
var milliSecondsDay = 24 * 60 * 60 * 1000;


This is a constant. Use all caps and underscore to indicate such. If you can use ES6, use const instead of var.

var currentDate = new Date();
var currentStamp = currentDate.getTime(); 


Use Date.now() to get the current timestamp. This saves you from creating a Date object.

return day.slice(-2) + separator + month.slice(-2) + separator + year;


Appears that you are using zero-padding. Create a separate function for it to make it reusable.

getDayBeforeIn


You aren't really returning the day, you're returning a Date object. Either you rename this to reflect the return value, like getDateBeforeIn or return a representation of the day of the week.