  We report on an improved systematic evaluation of the JILA SrI optical
lattice clock, achieving a nearly identical systematic uncertainty compared to
the previous strontium accuracy record set by the JILA SrII optical lattice
clock (OLC) at $2.1 \times 10^{-18}$. This improves upon the previous
evaluation of the JILA SrI optical lattice clock in 2013, and we achieve a more
than twenty-fold reduction in systematic uncertainty to $2.0 \times 10^{-18}$.
A seven-fold improvement in clock stability, reaching $4.8 \times
10^{-17}/\sqrt{\tau}$ for an averaging time $\tau$ in seconds, allows the clock
to average to its systematic uncertainty in under 10 minutes. We improve the
systematic uncertainty budget in several important ways. This includes a novel
scheme for taming blackbody radiation-induced frequency shifts through active
stabilization and characterization of the thermal environment, inclusion of
higher-order terms in the lattice light shift, and updated atomic coefficients.
Along with careful control of other systematic effects, we achieve low temporal
drift of systematic offsets and high uptime of the clock. We additionally
present an improved evaluation of the second order Zeeman coefficient that is
applicable to all Sr optical lattice clocks. These improvements in performance
have enabled several important studies including frequency ratio measurements
through the Boulder Area Clock Optical Network (BACON), a high precision
comparison with the JILA 3D lattice clock, a demonstration of a new all-optical
time scale combining SrI and a cryogenic silicon cavity, and a high sensitivity
search for ultralight scalar dark matter.
