JIRA is blocked by burndown chart calculating (wrong BURNDOWN_START_DATE setting)

Symptom

When accessing burndown chart for a specific version, JIRA instnace is blocked and CPU usage is maxing out.

Cause

Through thread dumps, I found that thread never jumped out of the below loop in com.pyxis.greenhopper.charts.GHChartUtil numberOfWorkingDays() function.

while (end.after(begin))
        {
            begin.add(GregorianCalendar.DAY_OF_YEAR, 1);
            int day = begin.get(GregorianCalendar.DAY_OF_WEEK);
            if (day > minBound && day < maxBound && !nonWorkingDates.contains(begin.getTime()))
            {
                days++;
            }
        }

Then go to the database, found that in GreenHopper project configuration, the version has a wrong start date setting as below

  <entry>
    <string>BURNDOWN_START_DATE_10475</string>
    <long>-61826266800000</long>
  </entry>

Workaround

Reset version start date in Planning Board may resolve the problem.

So far I can't find the reason that may cause BURNDOWN_START_DATE being set as a negative value.