  RX J1856.5-3754 is the X-ray brightest among the nearby isolated neutron
stars. Its X-ray spectrum is thermal, and is reproduced remarkably well by a
black-body, but its interpretation has remained puzzling. One reason is that
the source did not exhibit pulsations, and hence a magnetic field
strength--vital input to atmosphere models--could not be estimated. Recently,
however, very weak pulsations were discovered. Here, we analyze these in
detail, using all available data from the XMM-Newton and Chandra X-ray
observatories. From frequency measurements, we set a 2-sigma upper limit to the
frequency derivative of \dot\nu<1.3e-14 Hz/s. Trying possible phase-connected
timing solutions, we find that one solution is far more likely than the others,
and we infer a most probable value of \dot\nu=(-5.98+/-0.14)e-16 Hz/s. The
inferred magnetic field strength is 1.5e13 G, comparable to what was found for
similar neutron stars. From models, the field seems too strong to be consistent
with the absence of spectral features for non-condensed atmospheres. It is
sufficiently strong, however, that the surface could be condensed, but only if
it is consists of heavy elements like iron. Our measurements imply a
characteristic age of about 4 Myr. This is longer than the cooling and
kinematic ages, as was found for similar objects, but at almost a factor ten,
the discrepancy is more extreme. A puzzle raised by our measurement is that the
implied rotational energy loss rate of about 3e30 erg/s is orders of magnitude
smaller than what was inferred from the H-alpha nebula surrounding the source.
