Navigation & Timekeeping · Projected milestone

Second redefined by optical clocks

Estimated year~2030

70% by 2035 · including earlier years

The year is an editorial timing estimate. The percentage applies to occurrence by the stated deadline. These estimates are not calibrated odds.

The SI second is redefined on optical atomic clocks, about 100 times more precise than the caesium standard.

What the forecast means

Milestone stage: milestone (legacy definition)

Completion criteria: Legacy milestone: refine the observable threshold before resolving.

This page describes a forecast, rather than an achieved result. Read the methodology and assumptions.

No individual source links are recorded for this milestone. Source coverage and review status vary across the atlas.

Dataset updated 2026-10-18. This individual record has not been separately re-reviewed.