Thanks for getting in touch!
What axis type are you using for your X-Axis?
As you mentioned dates but are assigning a SChartNumberRange rather than an SChartDateRange.
If you are using an SChartDateTimeAxis you can get the monthly tick labels by setting your X-Axis "majorTickFrequency" property to a month.
If you want to format the tickLabels you can use the "labelFormatter" property on your X-Axis.
Let me know if you have any questions.