I spent a good 4 hours trying to figure out how to get HighCharts to use a custom date in the tooltip without using the custom tooltip formatter. Geesh! Why is it so painful?? Anyways, here is a JSFiddle showing how you can add your own custom formats to HighCharts and then use them in the date formats on the tooltip. In this example I’ve added a %c and then I can use that anywhere in HighCharts that I want to use my custom date formatting.