cross-device clock sync

real ntp corrects your clock against a hierarchy of atomic time servers. this tool does the same math locally — two browser tabs ping-pong high-resolution timestamps at each other, then compute offset and round-trip time using the classic ntp algorithm. open this page in a second tab (or on a second device via the same origin), assign one as initiator and one as responder, and watch the offset converge as measurement noise averages out.

open this page in two tabs · assign one role to each · start both · the initiator accumulates data

• ntp is shockingly accurate. using multiple servers and statistical filtering it achieves 1–10 ms accuracy over public internet, and sub-millisecond accuracy on a LAN.
• clock offset matters for distributed systems. causality, distributed transactions, and event ordering all depend on clocks that agree to within a few hundred microseconds.
• browsers deliberately reduce timestamp precision (to ~1 ms, or ~0.1 ms with COOP/COEP headers) to mitigate Spectre-style timing side channels — which means this tool’s cross-tab numbers should always be treated as noisy.
• performance.now() is origin-relative, not wall-clock. to compare clocks across origins you must use Date.now() despite its coarser resolution.