score:1

Use the format option in the axis declaration instead. http://c3js.org/reference.html#axis-x-tick-format

i.e.

axis: {
    x: {
          tick: {
            multiline: false,
            rotate: 75,
            format: '%d/%m/%Y', // NEW LINE
          },
          type: 'timeseries'
        },
    ... etc

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