widgy,

You can use xValueFormatString property of dataSeries to define how x-values must be formatted before they appear on the indexLabel or toolTip. You can format numbers and date time values using this property. For example if you set xValueFormatString as xValueFormatString: "DD/MMM/YYYY HH:mm:ss" then the x-value appearing in the toolTip will be formatted as shown in the image below.

Please refer to this JSFiddle for a working example with code.

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Anjali