Thorsten Kukuk demonstrates
that we are not done with year-2038 problems yet.

The general statement so far has always been that on 64bit systems
with a 64bit time_t you are safe with respect to the Y2038
problem. But glibc uses for compatibility with 32bit userland
applications 32bit time_t in some places even on 64bit systems.

One of those places is the utmp file.
The post includes a proposal for solving the problem by getting rid of
utmp entirely.

Source: LWN.net – Kukuk: Y2038, glibc and utmp/utmpx on 64bit architectures