  We report results of X-ray timing analyses for the low-field magnetar
CXOU~J164710.2$-$455216 which exhibited multiple outbursts. We use data taken
with NICER, NuSTAR, Chandra, and Neil-Gehrels-Swift telescopes between 2017 and
2018 when the source was in an active state. We perform semi-phase-coherent
timing analyses to measure the spin parameters and a spin-inferred
magnetic-field strength ($B_s$) of the magnetar. Using a semi-phase-coherent
method, we infer the magnetic field strengths to be $3-4\times 10^{13}\rm \ G$
at the observation period ($\sim$MJD 58000), and by comparing with previous
frequency measurements (MJD 54000) a long-term average value of $B_s$ is
estimated to be $\approx4\times 10^{13}\rm \ G$. So this analysis may add
CXOU~J164710.2$-$455216 to the ranks of low-field magnetars. The inferred
characteristic age ($\tau_c$) is 1--2 Myr which is smaller than the age of
Westerlund~1, so the magnetar's association with the star cluster is still
secure. For the low dipole field and the large age, recent multiple outbursts
observed from the source are hard to explain unless it has strong magnetic
multipole components. We also find timing anomalies around outburst epochs,
which suggests that there may be spin-down torque applied to the magnetar near
the epochs as was proposed in magnetar models.
