Accepted answer

In case anyone is interested :

It was this custom function that I made : addMonths(startMonth, monthIndex) which caused all the problems. There was absolutely no output from IE for me to know this was happening, it took a lot of elbow grease to figure it out. Instead I just installed moment.js and used this function instead to increment months, and display the value in milliseconds which highcharts will respect :

moment(startMonth, "YYYY-MM-DD").add(monthIndex, 'months').valueOf()

Note that the extra parameter with the date-format needs to be specified, or, once again, this will work in all browsers EXCEPT IE.

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