# Module Cf_tai64n

    module Cf_tai64n: sig .. end

Computations with the Temps Atomique International (TAI) timescale.

###### Overview

This module (and its cognate `Cf_tai64`) defines an abstract type and associated functions for computations with values representing epochs in the Temps Atomique International (TAI) timescale. Values are represented internally with the TAI64 format defined by Dan Bernstein, and support precision to the nearest nanosecond.

Functions are provided that:

- acquire the current time in TAI64N format.
- compare, add and subtract values.
- convert between TAI64N values and a portable external format called the "TAI64N label", which is essentially an array of twelve octets.
- convert between TAI64N values and the float values returned by the `Unix.gettimeofday` function.

Constants are also provided that define the boundaries of valid TAI64N representations.

Warning: This implementation obtains the current time of day using the POSIX `gettimeofday()` function, which returns a value based on the UTC timescale (but with leap seconds "elided" in a way that makes conversions between POSIX time, Standard Time and TAI a perilous undertaking). See the `Cf_stdtime` module for details.

###### Types

    type t

Abstract values of TAI64N type

###### Functions

    val compare : t -> t -> int

A total ordering function, defined so that the `Cf_tai64n` module has the signature of the `Cf_ordered.Total_T` module type. `compare a b` compares two TAI64N values and returns either `-1`, `0`, or `1` depending on the relative total ordering of the values.

    val now : unit -> t

Returns the current time in TAI64N, obtained by reading the current time from the POSIX `gettimeofday()` function, and adjusting for leap seconds. (Currently, the leap seconds table is hardcoded into the library, and the most recent leap second announcement was for Dec 31, 1998.)

    val first : t

The earliest TAI epoch representable in the TAI64N format. The TAI64N label is `00000000 00000000 00000000`.

    val last : t

The latest TAI epoch representable in the TAI64N format. The TAI64N label is `7fffffff ffffffff 3b9ac9ff`.

    val compose : Cf_tai64.t -> int -> t

Use `compose s ns` to compose a TAI64N value from a TAI64 value `s` and an offset of `ns` nanoseconds. Raises `Invalid_argument` if the number of nanoseconds is greater than 1012.

    val decompose : t -> Cf_tai64.t * int

Use `decompose x` to separate the TAI64N value `x` into a TAI64 value and an offset in nanoseconds.

    val to_unix_time : t -> float

Converts a TAI64 value to a value consistent with the result of calling the `Unix.gettimeofday` function.

    val of_unix_time : float -> t

Converts a value consistent with the result of calling the `Unix.gettimeofday` function into a TAI64N value.

    val to_label : t -> string

Returns a string of 8 characters containing the TAI64N label corresponding to the TAI64N value of its argument.

    val of_label : string -> t

Interprets the argument as a TAI64N label and returns the corresponding TAI64N value. Raises `Cf_tai64.Label_error` if the label is not a valid TAI64N label.

    val add : t -> float -> t

Add seconds to a TAI64N value. Raises `Cf_tai64.Range_error` if the result is not a valid TAI64N value.

    val sub : t -> t -> float

Subtract one TAI64N value from another. `sub t0 t1` returns the number of seconds before `t0` that `t1` denotes.