UUID decoder: version, variant and creation time

What you can extract from a UUID

The available data depends on the version. In v4 it is only the version, the variant and 122 random bits: neither time nor hardware is encoded in it. In v7 the first 48 bits are Unix time in milliseconds, followed by the random part. In v1 and v6 time is counted in 100-nanosecond intervals, and the last 48 bits are the node. v3 and v5 are deterministic: they are built from a namespace and a name, but the original name cannot be recovered from the hash.

Time in v1 and v7

v1 counts time from 15 October 1582, the date the Gregorian calendar was introduced. Internally it is a 60-bit value with a 100-nanosecond step; when decoded it is converted to Unix time and a regular date. v7 is simpler: the first six bytes hold the number of milliseconds since 1 January 1970, so decoding comes down to reading that number.

Worth remembering: the time in v1 and v7 is the moment of generation, not the moment the record is written to the database. In a distributed system, client and server clocks can drift by seconds.

Why it matters

A decoder is a diagnostic tool. It is used to verify that an application really generates v7 and not v4, to make sense of logs during incidents, to check the order of events, and to see whether creation time leaks into public links. For v1 there is another reason for attention — the node: historically it held a MAC address, and such a UUID should not be exposed in public addresses.

Decoding runs in the browser. Neither the UUID nor the result is sent anywhere.