See http://pobox.com/~djb/libtai/tai64.html for the latest version of
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TAI64
D. J. Bernstein, djb@pobox.com
19980222


1. Introduction

Today's UNIX systems, and many Internet protocols, store times as signed
32-bit integers representing the number of seconds since the beginning
of 1970. This format will expire in 2038.

Other systems, and other protocols, store times as unsigned 32-bit
integers representing the number of seconds since 1900. This format will
expire in 2036.

This document defines a 64-bit format, TAI64, suitable for use in new
systems and protocols. TAI64 represents a few hundred billion years of
real time with 1-second precision.


2. The TAI64 timescale

TAI stands for Temps Atomique International, the current international
real time standard. One TAI second is defined as the duration of
9192631770 periods of the radiation corresponding to the transition
between the two hyperfine levels of the ground state of the cesium atom.
(TAI also specifies a frame of reference; further discussion of
relativity is outside the scope of this document.)

TAI64 defines integer labels, between 0 and 2^64, for particular seconds
of real time. If 2^62 <= n < 2^63 then label n refers to the TAI second
beginning exactly n - 2^62 seconds after the beginning of 1970 TAI. If
0 <= n < 2^62 then label n refers to the TAI second beginning exactly
2^62 - n seconds before the beginning of 1970 TAI.

For example, label 4611686018427387904 refers to the second beginning at
the start of 1 January 1970 TAI. Label 4611686018427387903 refers to the
previous second; label 4611686018427387905 refers to the next second.

Labels 2^63 and larger are currently reserved for future extensions.
Under many cosmological theories, the labels under 2^63 are adequate to
cover the entire expected lifespan of the universe; in this case no
extensions will be necessary.


3. The TAI64 format

A TAI64 label is stored or communicated in eight 8-bit bytes in
big-endian format. This means that bytes b0 b1 b2 b3 b4 b5 b6 b7
represent the label b0 * 2^56 + b1 * 2^48 + b2 * 2^40 + ... + b7.

For example, bytes 64 0 0 0 0 0 0 0 represent the second that began
1970 TAI. Bytes 63 255 255 255 255 255 255 255 represent the previous
second; bytes 64 0 0 0 0 0 0 1 represent the next second.

Bytes 64 0 0 0 52 53 54 55 represent 3 October 1997 18:15:19 TAI, also
known as 3 October 1997 18:14:48 UTC.


4. Handling fractional seconds

One-second precision is inadequate for many applications. This section
defines 96-bit and 128-bit formats, TAI64N and TAI64NA, with nanosecond
and attosecond precision respectively.

A TAI64N label refers to a TAI nanosecond. It has two parts: (1) a
second count, meaning a TAI64 label; (2) a nanosecond count, meaning an
integer between 0 and 999999999 inclusive. It is stored in twelve 8-bit
bytes, with the TAI64 label followed by the nanosecond count in
big-endian format.

A TAI64NA label refers to a TAI attosecond. It has three parts: (1) a
second count; (2) a nanosecond count; (3) an attosecond count, meaning
an integer between 0 and 999999999 inclusive. It is stored in sixteen
8-bit bytes, with the TAI64 label followed by the nanosecond count in
big-endian format and then the attosecond count in big-endian format.