We have analyzed XMM-Newton and Chandra observations of the transient magnetar XTE J1810–197 spanning more than 11 years , from the initial phases of the 2003 outburst to the current quiescent level . We investigated the evolution of the pulsar spin period and we found evidence for two distinct regimes : during the outburst decay , \dot { \nu } was highly variable in the range - ( 2 - 4.5 ) \times 10 ^ { -13 } Hz s ^ { -1 } , while during quiescence the spin-down rate was more stable at an average value of -1 \times 10 ^ { -13 } Hz s ^ { -1 } . Only during \sim 3000 days ( from MJD 54165 to MJD 56908 ) in the quiescent stage it was possible to find a phase-connected timing solution , with \dot { \nu } = -4.9 \times 10 ^ { -14 } Hz s ^ { -1 } , and a positive second frequency derivative , \ddot { \nu } = 1.8 \times 10 ^ { -22 } Hz s ^ { -2 } . These results are in agreement with the behavior expected if the outburst of XTE J1810–197 was due to a strong magnetospheric twist .