Signed time

Any clock on the web can show a number. A signed timestamp lets you prove that a specific time came from clocknow's server and was not edited or replayed: the server signs it with an Ed25519 private key that never leaves the server, and anyone can check the signature with the published public key.

Request

GET https://api.clocknow.net/v1/signed?nonce=<random>. The nonce is optional but recommended: up to 64 characters of A–Z a–z 0–9 - _. Because it is signed too, a fresh random nonce proves the answer was made for your request.

Response

{
  "v": 1,
  "kid": "2026-11",
  "t": 1793929327182,
  "radius_ms": 100,
  "nonce": "q0V3kq8ZxV0Yp1rS4mJ2Hw",
  "server": "fly-sin-1a2b3c",
  "sig": "…86 base64url characters…"
}

t is the server's time in Unix milliseconds and radius_ms is the server's own error bound for its clock. sig is the base64url Ed25519 signature of these exact bytes, joined with newlines:

clocknow-signed-time-v1
kid=2026-11
t=1793929327182
radius=100
nonce=q0V3kq8ZxV0Yp1rS4mJ2Hw
server=fly-sin-1a2b3c

Public keys

GET https://api.clocknow.net/.well-known/clocknow-keys.json lists the current key and retired ones, each as a raw 32-byte Ed25519 public key in base64url. Keys are rotated with a new kid; retired keys stay listed so old timestamps can still be checked.

Verify in Python

import base64, json, secrets, urllib.request
from cryptography.hazmat.primitives.asymmetric.ed25519 import Ed25519PublicKey

def b64(s): return base64.urlsafe_b64decode(s + "=" * (-len(s) % 4))

nonce = secrets.token_urlsafe(16)
st = json.load(urllib.request.urlopen(f"https://api.clocknow.net/v1/signed?nonce={nonce}"))
keys = json.load(urllib.request.urlopen("https://api.clocknow.net/.well-known/clocknow-keys.json"))
key = next(k for k in keys["keys"] if k["kid"] == st["kid"])
msg = (f"clocknow-signed-time-v1\nkid={st['kid']}\nt={st['t']}\nradius={st['radius_ms']}"
       f"\nnonce={st['nonce']}\nserver={st['server']}").encode()
assert st["nonce"] == nonce
Ed25519PublicKey.from_public_bytes(b64(key["public_key"])).verify(b64(st["sig"]), msg)
print("verified:", st["t"], "±", st["radius_ms"], "ms")

Or verify in your browser on the atomic clock page.

What it does not prove

A signature proves who said the time and that it was not changed. The network delay between the server and you is not signed, so for precise synchronisation measure the round trip yourself, as the clocks on this site do.