interface IgxSummaryResult {
  key: string;
  label: string;
  summaryResult: any;
}

class CustomDateSummary extends IgxDateSummaryOperand {
  constructor() {
    super();
  }
  public operate(data: any[], allData: any[], fieldName: string, groupRecord: IGroupByRecord): IgxSummaryResult[] {
    const result = super.operate(data, allData, fieldName, groupRecord);
    result.push({
      key: "deadline",
      label: "Deadline Date",
      summaryResult: this.calculateDeadline(data);
    });
    return result;
  }
}
this.grid.getColumnByName('ColumnName').summaries = CustomDateSummary;

IgxSummaryOperand.count(dataSource);

IgxDateSummaryOperand.earliest(data);

IgxDateSummaryOperand.latest(data);