Absolute timing as a function of MJD using Crab-pulsar monitoring data. Shown are the offsets of the main X-ray peak with respect to the main radio peak in the Crab pulse profile in time units (left scale) and phase units (right scale). The coloured lines give the mean values for XMM-Newton (solid black line, this work), Chandra (green dashed line), RXTE (red dot line) and INTEGRAL (blue dashed-dot line), respectively, all taken from the literature. The yellow area indicates the standard deviation of the XMM-Newton data points. The superscript numbers near to each XMM-Newton data point give the XMM-Newton revolution in which the observation was carried out.