How to do international date and time formatting

We're using Highcharts for visualize continuous measurement data on our website.

As the customers sit around the world we need some proper date and time formatting to show the time and date on the x-axis in a locally common format.

What is the best way to detect which region the customer is in and is there any library or list according to ISO 8601 that can help us? (I don't want to rely on IP region detection, by the way).

Thanks.