Abstract: Atomic clocks are arguably the most accurate scientific instruments ever built. Modern clocks are astonishing timepieces guaranteed to keep time within a second over the age of the Universe. I will review the remarkable progress in atomic timekeeping and discuss a variety of applications of atomic clocks as quantum sensors. These applications range from measuring electric fields and geoids to dark matter and exotic physics searches. Massive entanglement of atomic ensembles offers an intriguing potential for improving the sensitivity even further. I will present our proposal on entangling millions of atoms with Rydberg gates.