  A proton is known for its longevity, but what is its lifetime? While many
Grand Unified Theories predict the proton decay with a finite lifetime, we show
that the Standard Model (SM) and some versions of Ultra Unification (which
replace sterile neutrinos with new exotic gapped/gapless sectors, e.g.,
topological or conformal field theory under global anomaly cancellation
constraints) with a discrete baryon plus lepton symmetry permit a stable
proton. For the 4d SM with $N_f$ families of 15 or 16 Weyl fermions, in
addition to the continuous baryon minus lepton U(1)$_{\bf B - L}$ symmetry,
there is also a compatible discrete baryon plus lepton $\mathbb{Z}_{2N_f, \bf B
+ L}$ symmetry. The $\mathbb{Z}_{2N_f, \bf B + L}$ is discrete due to the ABJ
anomaly under the BPST SU(2) instanton. Although both U(1)$_{\bf B - L}$ and
$\mathbb{Z}_{2N_f, \bf B + L}$ symmetries are anomaly-free under the dynamical
SM gauge field, it is important to check whether they have mixed anomalies with
the gravitational background field and higher symmetries (whose charged objects
are Wilson electric or 't Hooft magnetic line operators) of SM. We can also
replace the U(1)$_{\bf B - L}$ with a discrete variant $\mathbb{Z}_{4,X}$ for
$X \equiv 5({\bf B - L})-\frac{2}{3} {\tilde Y}$ of electroweak hypercharge
${\tilde Y}$. We explore a systematic classification of candidate perturbative
local and nonperturbative global anomalies of the 4d SM, including all these
gauge and gravitational backgrounds, via a cobordism theory, which controls the
SM's deformation class. We discuss the proton stability of the SM and Ultra
Unification in the presence of discrete ${\bf B + L}$ symmetry protection, in
particular (U(1)$_{\bf B - L} \times \mathbb{Z}_{2N_f,\bf B +
L})/{\mathbb{Z}_2^{\rm F}}$ or $(\mathbb{Z}_{4,X} \times \mathbb{Z}_{2N_f, \bf
B + L})/{\mathbb{Z}_2^{\rm F}}$ symmetry with the fermion parity
$\mathbb{Z}_2^{\rm F}$.
