I have a grid similar patterned after the Live Group Summary example in the 3.x showcase. Two of my columns contain dates and I would like to be able to have the latest date from each column appear in the summary row for each group. I wrote this code to override the caculate method:

SummaryColumnConfig<GranuleThroughputByProduct, Date> lastIngestedCol = new SummaryColumnConfig<GranuleThroughputByProduct, Date>(
                props.lastIngestedTime(), 100, "Last Ingested Time");
        lastIngestedCol.setCell(new DateCell(DateTimeFormat.getFormat(CommonConstants.DATE_TIME_ISO8601)));
        lastIngestedCol.setSummaryType(new SummaryType<GranuleThroughputByProduct, Date, Number>() {
            public Number calculate(
                    List<GranuleThroughputByProduct> m,
                    ValueProvider<GranuleThroughputByProduct, Date> valueProvider) { 
                int dummyVal = 21;
                Date latest = null;
                for (int i = 0; i < m.size(); i++) {
                    GranuleThroughputByProduct t = m.get(i);
                     if (latest == null) {
                         latest = t.getLastIngestedTime();
                     } else if (latest.before(t.getLastIngestedTime())) {
                         latest = t.getLastIngestedTime();
                return dummyVal;
When I do this I get the error:
Caused by: java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Number
    at com.sencha.gxt.widget.core.client.grid.GroupSummaryView.calculate(GroupSummaryView.java:90)
Now the calculate method itself doesn't tip you off that it needs to return a Number but the render method obviously needs to take a Number to work correctly. This is where I assume the problem lies.

Is there anyway to get this to work? It seems that Summary columns should be compatible with more date types than just Numbers since it is a fairly common query to ask for the latest date (for example in a SQL query or excel spreadsheet).