While it would be better to manually unpublish the form or to add a Custom HTML explaining that form submissions will not be accepted before/after a certain time, it's possible to use the following JavaScript to essentially scrub the form of all content if today's date is not between a defined start and end date:

$(document).ready(function(){
  
    // The start date (July 1, 2021) in YYYY-MM-DD format
    var startDate = new Date('2021-07-01');
      
    // The end date (July 31, 2021) in YYYY-MM-DD format
    var endDate = new Date('2021-07-31');
    
    // Today's date
    var today = new Date();
  
    // If today is before the start date, replace the form's content with a message.
    if (today < startDate){
      document.body.innerHTML = "<p>This form will become available on July 1, 2021.</p>";          
    }
    
    // If today has passed the end date, replace the form's content with a message.
    if (today >=endDate){
      document.body.innerHTML = "<p>This form is now closed.</p>";          
    }
}); 

Some caveats here is that JavaScript uses the client's machine for the current date. So if they could feasibly change their device's date setting to circumvent it.