# Module Cf_tai64

    module Cf_tai64: sig .. end

Computations with the Temps Atomique International (TAI) timescale.

Overview

This module 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 second.

Functions are provided that:

- acquire the current time in TAI64 format.
- compare, add and subtract values.
- convert between TAI64 values and a portable external format called the "TAI64 label", which is essentially an array of eight octets.
- convert between TAI64 values and the float values returned by the `Unix.time` function.

Constants are also provided that define the boundaries of valid TAI64 representations.

Warning: This implementation obtains the current time of day using the POSIX `time()` 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 TAI64 type

Exceptions

    exception Range_error

Result not representable in TAI64 format.

    exception Label_error

Input is not a valid TAI64 label

Functions

    val compare : t -> t -> int

A total ordering function, defined so that the `Cf_tai64` module has the signature of the `Cf_ordered.Total_T` module type. `compare a b` compares two TAI64 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 TAI64, obtained by reading the current time from the POSIX `time()` 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 TAI64 format. The TAI64 label is `0000000000000000`.

    val last : t

The latest TAI epoch representable in the TAI64 format. The TAI64 label is `7fffffffffffffff`.

    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.time` function into a TAI64 value.

    val to_label : t -> string

Returns a string of 8 characters containing the TAI64 label corresponding to the TAI64 value of its argument.

    val of_label : string -> t

Interprets the argument as a TAI64 label and returns the corresponding TAI64 value. Raises `Label_error` if the label is not a valid TAI64 label.

    val add : t -> int -> t

Add seconds to a TAI64 value. Raises `Range_error` if the result is not a valid TAI64 value.

    val add_int32 : t -> int32 -> t

Add seconds to a TAI64 value. Raises `Range_error` if the result is not a valid TAI64 value.

    val add_int64 : t -> int64 -> t

Add seconds to a TAI64 value. Raises `Range_error` if the result is not a valid TAI64 value.

    val sub : t -> t -> int64

Subtract one TAI64 value from another. `sub t0 t1` returns the number of seconds before `t0` that `t1` denotes.