get_oldest_starting_date() {
        if (!this.tasks.length) return new Date();
        return this.tasks
            .map((task) => task._start)
            .reduce((prev_date, cur_date) =>
                cur_date <= prev_date ? cur_date : prev_date,
            );
    }
Gets the oldest starting date from the list of tasks

@returns Date
@memberof Gantt