计算运行时长

        public string GetTickCount(DateTime? startTime, DateTime? endTime)
        {
            if (!startTime.HasValue)
                return null;

            string tickCount = null;

            if (!endTime.HasValue)
            {
                endTime = DateTime.Now;
            }

            if (startTime.Value > endTime.Value)
                throw new ValidationException("开始时间不能大于结束时间");

            var totalSeconds = (endTime.Value - startTime.Value).TotalSeconds;
            var days = Math.Truncate(totalSeconds / 60 / 60 / 24);
            var hours = Math.Truncate(totalSeconds / 60 / 60 % 24);
            var minutes = Math.Truncate(totalSeconds / 60 % 60);
            var seconds = Math.Truncate(totalSeconds % 60);

            if (days > 0)
            {
                tickCount += $"{days}天";
            }

            if (hours > 0)
            {
                tickCount += $"{hours}小时";
            }

            if (minutes > 0)
            {
                tickCount += $"{minutes}分钟";
            }

            if (seconds > 0)
            {
                tickCount += $"{seconds}秒";
            }

            return tickCount;
        }