Yes you're correct about the datetime axis' minimum and maximum values default to the earliest and last dates in your data set.
You should be able to create a new range and set that as your chart's defaultRange (for datetime axes you would use an SChartDateRange) as follows:
_chart.xAxis.defaultRange = [[SChartDateRange alloc] initWithDateMinimum:[self dateFromString:@"01-02-2015"] andDateMaximum:[self dateFromString:@"01-03-2015"]];
Say you had data for 01 Feb to 01 April, this would result in the x axis adjusting to fit just the data from 01 Feburary to 01 March initially, with the user being able to pan and see the remaining datapoints (provided gesture panning is enabled of course!).
I hope that helps!