anchor.diff:          <div class="label">
               <form:label path="period.startDay">Start day</form:label>
           </div>
  -        <div class="tip">
  +        <div class="tip" id="startDayText">
               The relative day of the start of this period.  This may be positive, negative, or zero.
           </div>
           <div class="value">
  
positive.code:         <div class="label">
              <form:label path="period.duration.quantity">Duration</form:label>
          </div>
          <div class="tip">
              The length of a single repetition of this period.  This must be a positive number,
              and may be expressed in days or weeks.
          </div>
  
negative.code:             else return q;
          }
  
          function resetElement(elementId, text, color) {
              var element = document.getElementById(elementId);
              element.style.color = color;
              element.innerHTML = text;
          }
  
          function isCorrectStartDay() {
              var startDay = document.getElementById("period.startDay").value;
              var isDataCorrect = true;
              if(startDay.length <= 0 || (isNaN(startDay*1))) {
                  isDataCorrect = false;
                  resetElement("periodError",
                    "ERROR: Start Day must be positive, negative, or zero.", "red");
              }
              return isDataCorrect;
          }
  
          function isCorrectDuration() {
              var duration = document.getElementById("period.duration.quantity").value;
              var isDataCorrect = true;
              if (duration.length <=0 || ((isNaN(duration*1)) || (duration <=0))) {
                  isDataCorrect = false;
                  resetElement("periodError",
                    "ERROR: Duration must be a positive number.", "red");
              }
              return isDataCorrect;
          }
  
          function isCorrectRepetitions() {
              var repetitions = document.getElementById("period.repetitions").value;
              var isDataCorrect = true;
              if (repetitions.length <=0 || ((isNaN(repetitions*1)) || (repetitions <=0))) {
                  isDataCorrect = false;
                  resetElement("periodError",
                                    "ERROR: Repetitions must be a positive number.", "red");
              }
              return isDataCorrect;
          }
  
          function isCorrectInput() {
              var isDataCorrect = true;
              isDataCorrect = isCorrectStartDay();
              if(!isDataCorrect) { return isDataCorrect; }
              isDataCorrect = isCorrectDuration();
              if(!isDataCorrect) { return isDataCorrect; }
              return isCorrectRepetitions();
          }
  
          Element.observe(window, "load", function() {
              $$("#period-form input").each(function(elt) {
                  Element.observe(elt, "change", updateSummary)
  