Comparing the date entered to ensure it is between today and 4 weeks from now

For future questions about scripts in Acrobat you'll be better off using this forum: JavaScript

JavaScript is the same in either instance, but what you can do with it and what contains it may allow it to do more or less. Acrobat is an example of an environment where JavaScript has extended capabilities to work with the interface.

Here is a verbose, really simple way to determine the number of days between 2 dates, down to the microsecond. On the JSFiddle example I just changed how I am displaying the result for that site.

var one_day_in_milliseconds = 1000*60*60*24; // 86400000ms
// date you want to compare
var compare = new Date("09/01/2018")
// todays date (per local machine)
var today = new Date();
// use Date.getTime() to get millisecond comparison difference
var difference = compare.getTime() - today.getTime();
// what to display
var message;
if ( difference > 0 ) {
    // difference in days, ignoring partial days (no decimal)
    var days = Math.floor( difference / one_day_in_milliseconds );
    message = days + " day(s) is";
    // 28 days is 4 weeks
    if ( days <= 28 ) {
          message += " within 4 weeks from now.";
    } else {
          message += " further than 4 weeks from now.";
    }
} else {
    //the compared date is in the past, automatically this isn't desirable
    message = "The date being compared is not valid, it is in the past.";
}
alert( message );

JSFiddle example

There are quite a few ways to do this and this is only verbose to make it very clear what is going on. Searching around for JavaScript date comparison will yield quite a bounty of different ways to do it. This is also ES5 for compatibility.

Note: If you wanted to include partial days, change line 18 from "Math.floor" to "Math.ceil"