Allow labels on datetime axis to be modified prior to formatting
Using local time for a datetime axis is usually a bad idea, because when daylight saving is introduced parts of the x-axis would overlap and other parts would have gaps. So data points on the axis are best plotted using UTC. However, it can then be useful to label them in local time. It might also be that the local time required is different to the PC local time, if you are supporting a Italian user from the UK, for example. These requirements could easily be supported by modifying the dateFormat function to call a user-defined adjustment function that could add or subtract several hours as appropriate prior to creating the label.
Also, you can add or substract hours in the xAxis.label.formatter.