anchor.diff:                  numberOfDaysInUnit =1;
               }
               totalDays = reps * duration * numberOfDaysInUnit;
  -            Element.update("summary-days", totalDays + " day" + (totalDays != 1 ? "s" : ""))
  -            highlightOdd("summary-days")
  -
  -            if (duration > 1) {
  -                Element.update("summary-day-range", "(" + startDay + " to " + (startDay + duration * reps - 1) + ")")
  +            if (isCorrectInput()) {
  +                Element.update("summary-beginning", " The configured period will last for ")
  +                Element.update("summary-days", totalDays + " day" + (totalDays != 1 ? "s" : ""))
  +                highlightOdd("summary-days")
  +
  +                resetElement("periodError","");
  +                if (duration > 1) {
  +                    Element.update("summary-day-range", "(" + startDay + " to " + (startDay + duration * numberOfDaysInUnit * reps - 1) + ")")
  +                } else {
  +                    Element.update("summary-day-range", "(day " + startDay + ")")
  +                }
               } else {
  -                Element.update("summary-day-range", "(day " + startDay + ")")
  +                disableSummaryFields()
               }
               highlightOdd("summary-day-range")
   
  -            if (reps > 1 && duration >= 1) {
  +            if (reps > 1 && duration >= 1 && isCorrectStartDay()) {
                   var sameDay = [startDay]
                   while (sameDay.length < reps) {
                       sameDay.push(sameDay.last() + duration * numberOfDaysInUnit)
  
positive.code:             </div>
              <div class="row odd" id="summary">
                  <div class="label">Summary</div>
                  <div class="value">
                      The configured period will last for <span id="summary-days">1 day</span>
                      <span id="summary-day-range">(day 1)</span>.
              <span id="summary-repetitions">
                  It will have the same events on days <span id="summary-event-days"></span>.
              </span>
  
negative.code:                 </div>
                  <div class="value">
                      <form:input path="period.duration.quantity" size="3" maxlength="3"/>
                      <form:select path="period.duration.unit">                                   
                          <form:options items="${durationUnits}"/>
                      </form:select>
                  </div>
              </div>
              <div class="row even">
                  <div class="label">
                      <form:label path="period.repetitions">Occurrences</form:label>
                  </div>
                  <div class="tip" id="repetitionsText">
                      The number of times the days of this period will occur.
  