Time-Based UUID — v1, v6, and v7 Compared

A time-based UUID embeds a timestamp so that values generated later sort after values generated earlier. Three UUID versions include a timestamp: v1, v6, and v7. Only v7 is recommended for new work.

UUID v1 (Deprecated)

UUID v1 was the original time-based format, defined in RFC 4122 (2005) and deprecated by RFC 9562 (2024).

Bit layout:

time_low     (32 bits) — low 32 bits of 60-bit timestamp
time_mid     (16 bits) — middle 16 bits of timestamp
time_hi_ver  (16 bits) — high 12 bits + 4-bit version (1)
clock_seq    (14 bits) — sequence number + 2-bit variant
node         (48 bits) — MAC address or random bytes

Problems with v1:

1. The timestamp fragments are stored in a non-sequential field order — sorting v1 UUIDs lexicographically does NOT give chronological order
2. The node field originally contained the host MAC address — a machine fingerprint and privacy leak
3. The 100-nanosecond epoch (October 15, 1582) is unusual and error-prone

Generating v1 (only for legacy compatibility):

import uuid
# uuid.uuid1() still works but is deprecated for new use
legacy_id = uuid.uuid1()

UUID v6 (Reordered v1)

UUID v6 reorders the timestamp fields from v1 so the UUID is lexicographically sortable — the most significant bits come first:

time_high (32 bits) — high 32 bits of 60-bit timestamp
time_mid  (16 bits) — middle 16 bits
time_low  (12 bits) — low 12 bits (after version nibble)
version   (4 bits)  — always 6
clock_seq (14 bits) — monotonic counter
variant   (2 bits)  — always 10
node      (48 bits) — MAC or random

v6 is a migration path for systems already on v1 — it is sortable, but still uses the same 1582 epoch and the node field. Prefer v7 for all new work.

UUID v7 (Recommended)

UUID v7 uses a clean Unix millisecond timestamp in the high 48 bits:

unix_ts_ms (48 bits) — Unix time in milliseconds
version    (4 bits)  — always 7
seq_hi     (12 bits) — monotonic counter or random
variant    (2 bits)  — always 10
rand_b     (62 bits) — cryptographically random

Why v7 wins:

• Unix epoch (January 1, 1970) — no unusual epoch arithmetic
• Lexicographically sortable — ORDER BY id gives creation order
• No MAC address — no privacy concern
• 74 bits of randomness — still collision-safe
• Native support: PostgreSQL 18 uuidv7(), Python 3.13 uuid.uuid7(), uuid npm v7()

import { v7 as uuidv7 } from 'uuid';
const id = uuidv7(); // 018fbe3a-4c5d-7b12-8abc-0123456789ab

import uuid
id = uuid.uuid7()  # Python 3.13+

Extracting the v7 Timestamp

function uuidV7ToDate(uuid) {
  const ms = parseInt(uuid.replace(/-/g, '').slice(0, 12), 16);
  return new Date(ms);
}

uuidV7ToDate('018fbe3a-4c5d-7b12-8abc-0123456789ab');
// → 2024-05-21T...

Version Comparison Table

Related Resources

• RFC 9562 §5.7 — UUID v7 specification
• RFC 9562 §5.6 — UUID v6 specification
• UUID Version — all 8 versions explained
• UUID byte layout — all 128 bits annotated
• RFC 9562 replaces RFC 4122 — what changed
• PostgreSQL 18 UUID functions