Hello James, 

Here is a solution using javascript taken from this thread https://answers.laserfiche.com/questions/75138/I-want-to-prevent-user-from-picking-a-date-greater-than-today  and modified.

For note, I have named my calendar class "calendarDate" as shown in the image below

From there, I have modified the javascript from the thread to only allow dates today and after.

$(function() {
  $(document).ready(function () {
    
   var todaysDate = new Date(); // Gets today's date
    
    // Max date attribute is in "YYYY-MM-DD".  Need to format today's date accordingly
    
    var year = todaysDate.getFullYear(); 						// YYYY
    var month = ("0" + (todaysDate.getMonth() + 1)).slice(-2);	// MM
    var day = ("0" + todaysDate.getDate()).slice(-2);			// DD

   	var minDate = (year +"-"+ month +"-"+ day); // Results in "YYYY-MM-DD" for today's date 
    
    // Now to set the max date value for the calendar to be today's date
    $('.calendarDate input').attr('min',minDate);
 
  });
});