DateTimeAxis Question

when using DateTimeAxis,I found a question.
bottomAxis[i] = new DateTimeAxis(AxisPosition.Bottom, "时间", "yyyy/MM/dd\nhh:mm:ss", DateTimeIntervalType.Milliseconds);
bottomAxis[i].MajorStep = 1.0 / 3600 / 24;
bottomAxis[i].MinorStep = 1.0 / 3600 / 24 / 10;
bottomAxis[i].ShowMinorTicks = true;
I want to set MajorStep=1 second and Minorstep=0.1 second.when I started,the DateTimeInterval was correct.
But when I set :
bottomAxis[i].Maximum = DateTimeAxis.ToDouble(DateTime.Now);
bottomAxis[i].Minimum = DateTimeAxis.ToDouble(DateTime.Now.AddMinutes(-1));
the DateTimeInterval become 9 Second per step.Why?
Please revert. Thanks in advance.

Regards