The world’s most accurate clocks lose
how much time over a billion years?

Less than one second.. Optical atomic clocks used at NIST are accurate to 1 in 10^18. GPS depends on them; so does modern civilisation, quietly.

Question

Accepted Answer

Less than one second. — Optical atomic clocks used at NIST are accurate to 1 in 10^18. GPS depends on them; so does modern civilisation, quietly.