The crosshair tooltip's text labels use the chart's axes to format the string.
Under the hood, it uses the method stringForId: which converts the given id (NSNumber NSDate e.t.c) to a string, taking into account the axis' labelFormatString. If one is not supplied then it uses a default label formatter.
If you're using a date axis, you can modify the value displayed by the tooltip by altering it's labelFormattring e.g.:
dateAxis.labelFormatString = @"d MM"
If you don't want to modify the axis' values and just the tooltip's label, then you should be able to subclass SChartCrosshairTooltip and override the method setResolvedDataPoint:fromSeries:fromChart:. You should then be able to set the text to whatever you wish.
I hope that helps!