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Invalid date in tooltip #976
It happens because a name is supplied for each point, and Highcharts tries to use the name instead of the date. But since this is a datetime axis, the point's key is passed through dateFormat.
The workaround is to remove the point names. If names are required for some other purpose than the tooltip, they can be given a different id and picked up in the formatters accordingly.
A possible fix for this would be to always pass in point.x to the tooltip formatter on datetime axes. It is a rare case though - normally there is no need to use point names on a datetime axis.
I believe the correct behaviour would be to not parse the 'name' key through dateFormat when on a datetime axis. If someone uses the name key on a point on a datetime axis it is because they want to display something else than the date on the tooltip.
In my case we are using the point name to display the milestone name on a project burndown graph. It makes more sence to display "Sprint 10 - Code Freeze" than it does to display "September 2012"