Steps to reproduce:

1. Create a page and add the HTML Macro with the following script:

   <script type="text/javascript">
   function workingDaysBetweenDates(startDate, endDate)
   { // Validate input if (endDate < startDate) return 0; // Calculate days between dates var millisecondsPerDay = 86400 * 1000; // Day in milliseconds startDate.setHours(0,0,0,1); // Start just after midnight endDate.setHours(23,59,59,999); // End just before midnight var diff = endDate - startDate; // Milliseconds between datetime objects var days = Math.ceil(diff / millisecondsPerDay); // Subtract two weekend days for every week in between var weeks = Math.floor(days / 7); var days = days - (weeks * 2); // Handle special cases var startDay = startDate.getDay(); var endDay = endDate.getDay(); // Remove weekend not previously removed. if (startDay - endDay > 1) days = days - 2; // Remove start day if span starts on Sunday but ends before Saturday if (startDay == 0 && endDay != 6) days = days - 1 // Remove end day if span ends on Saturday but starts after Sunday if (endDay == 6 && startDay != 0) days = days - 1 return days; }
   
   var FC = new Date("April 24, 2015");
   var Ship = new Date("May 30, 2015");
   var today = new Date();
   FCDaysLeft = workingDaysBetweenDates(today, FC);
   ShipDaysLeft = workingDaysBetweenDates(today, Ship);
   document.write("<H3>");
   document.write("Today is: " + today.toString().split(' ').splice(1,3).join(' ') +"<br />");
   document.write("There are " + FCDaysLeft + " working days left to FC <br />" );
   document.write("There are "+ ShipDaysLeft + " working days left to Ship <br />");
   document.write("</H3>");
   </script>
   

2. Save.
3. Now try to edit the page again.

Workaround:

Get the edit link manually, and add to the URL bar.