Using core plot for iPhone, drawing date on x axis

I have available an array of dictionary that contains NSDate and NSNumber values. I wanted to plot date on X axis. For plotting I need to supply xRanges to plot with some decimal values. I don't understand how can I supply NSDate values to xRange (low and length).

And what should be there in this method:

-(NSNumber *)numberForPlot:(CPPlot *)plot field:(NSUInteger)fieldEnum recordIndex:(NSUInteger)index

I mean how my date value will be returned as NSNumber? I think I should use some interval over there, but what should be the exact conversion? Can any one explain me what are the exact requirement to plot the date on xAxis?

I am plotting my plot in half of the view.