time/time-1.2.0

Represents an instance in time.

Description

A “time” is a single instant in time. It may explicitly specify the way time is represented (the “format”) and the “scale” which specifies the offset and scaling relation of the unit of time.

Specific emphasis is placed on supporting time scales (e.g. UTC, TAI, UT1, TDB) and time representations (e.g. JD, MJD, ISO 8601) that are used in astronomy and required to calculate, e.g., sidereal times and barycentric corrections.

Times may be represented as one of the following:

  • an object, with explicit value, and optional format, scale and location.

  • a string, in which case the format is guessed from across the unambiguous options (iso, byear, jyear, yday), and the scale is hardcoded to UTC.

In either case, a single time tag may be used to represent an n-dimensional array of times, using either an ndarray tag or inline as (possibly nested) YAML lists. If YAML lists, the same format must be used for all time values.

The precision of the numeric formats should only be assumed to be as good as an IEEE-754 double precision (float64) value. If higher-precision is required, the iso or yday format should be used.

Outline

Schema Definitions

This node must validate against any of the following:

  • This type is an object with the following properties:

    • value

      objectRequired

      The value(s) of the time.

      This node must validate against any of the following:

    • format

      format

      The format used to save the time in ASDF

      If not provided, the the format should be guessed from the string from among the following unambiguous options: iso, byear, jyear and yday.

    • base_format

      object

      The original format of the time object

      This node must validate against one of the following:

    • scale

      object

      The time scale (or time standard) is a specification for measuring time: either the rate at which time passes; or points in time; or both. See also [3] and [4].

      These scales are defined in detail in SOFA Time Scale and Calendar Tools.

      The supported time scales are:

      • utc: Coordinated Universal Time (UTC). This is the default time scale, except for gps, unix.

      • tai: International Atomic Time (TAI).

      • tcb: Barycentric Coordinate Time (TCB).

      • tcg: Geocentric Coordinate Time (TCG).

      • tdb: Barycentric Dynamical Time (TDB).

      • tt: Terrestrial Time (TT).

      • ut1: Universal Time (UT1).

      This node has no type definition (unrestricted)

    • location

      object

      Specifies the observer location for scales that are sensitive to observer location, currently only tdb. May be specified either with geocentric coordinates (X, Y, Z) with an optional unit or geodetic coordinates:

      • long: longitude in degrees

      • lat: in degrees

      • h: optional height

      This type is an object with the following properties:

Examples

Example ISO time:

!time/time-1.2.0 "2000-12-31T13:05:27.737"

Example year, day-of-year and time format time:

!time/time-1.2.0 "2001:003:04:05:06.789"

Example Besselian Epoch time:

!time/time-1.2.0 B2000.0

Example Besselian Epoch time, equivalent to above:

!time/time-1.2.0
  value: 2000.0
  format: byear

Example list of times:

!time/time-1.2.0
  ["2000-12-31T13:05:27.737", "2000-12-31T13:06:38.444"]

Example of an array of times:

!time/time-1.2.0
  value: !core/ndarray-1.1.0
    data: [2000, 2001]
    datatype: float64
  format: jyear

Example with a location:

!time/time-1.2.0
  value: 2000.0
  format: jyear
  scale: tdb
  location:
    x: !unit/quantity-1.2.0
      value: 6378100
      unit: !unit/unit-1.0.0 m
    y: !unit/quantity-1.2.0
      value: 0
      unit: !unit/unit-1.0.0 m
    z: !unit/quantity-1.2.0
      value: 0
      unit: !unit/unit-1.0.0 m

Internal Definitions

  • iso_time

    string
    No length restriction

    Must match the following pattern:

    [0-9]{4}-(0[1-9])|(1[0-2])-(0[1-9])|([1-2][0-9])|(3[0-1])[T ]([0-1][0-9])|(2[0-4]):[0-5][0-9]:[0-5][0-9](.[0-9]+)?

  • byear

    string
    No length restriction

    Must match the following pattern:

    B[0-9]+(.[0-9]+)?

  • jyear

    string
    No length restriction

    Must match the following pattern:

    J[0-9]+(.[0-9]+)?

  • yday

    string
    No length restriction

    Must match the following pattern:

    [0-9]{4}:(00[1-9])|(0[1-9][0-9])|([1-2][0-9][0-9])|(3[0-5][0-9])|(36[0-5]):([0-1][0-9])|([0-1][0-9])|(2[0-4]):[0-5][0-9]:[0-5][0-9](.[0-9]+)?

  • string_formats

    object

    This node must validate against any of the following:

  • array_of_strings

    array
    No length restriction

    Items in the array must be any of the following types:

  • format

    object

    The format of the time.

    The supported formats are:

    • iso: ISO 8601 compliant date-time format YYYY-MM-DDTHH:MM:SS.sss.... For example, 2000-01-01 00:00:00.000 is midnight on January 1, 2000. The T separating the date from the time section is optional.

    • yday: Year, day-of-year and time as YYYY:DOY:HH:MM:SS.sss.... The day-of-year (DOY) goes from 001 to 365 (366 in leap years). For example, 2000:001:00:00:00.000 is midnight on January 1, 2000.

    • byear: Besselian Epoch year, eg. B1950.0. The B is optional if the byear format is explicitly specified.

    • jyear: Julian Epoch year, eg. J2000.0. The J is optional if the jyear format is explicitly specified.

    • decimalyear: Time as a decimal year, with integer values corresponding to midnight of the first day of each year. For example 2000.5 corresponds to the ISO time 2000-07-02 00:00:00.

    • jd: Julian Date time format. This represents the number of days since the beginning of the Julian Period. For example, 2451544.5 in jd is midnight on January 1, 2000.

    • mjd: Modified Julian Date time format. This represents the number of days since midnight on November 17, 1858. For example, 51544.0 in MJD is midnight on January 1, 2000.

    • gps: GPS time: seconds from 1980-01-06 00:00:00 UTC For example, 630720013.0 is midnight on January 1, 2000.

    • unix: Unix time: seconds from 1970-01-01 00:00:00 UTC. For example, 946684800.0 in Unix time is midnight on January 1, 2000. [TODO: Astropy’s definition of UNIX time doesn’t match POSIX’s here. What should we do for the purposes of ASDF?]

    This node has no type definition (unrestricted)

  • other_format

    object

    The other formats supported by astropy.time: https://docs.astropy.org/en/latest/time/index.html#time-format

    This node has no type definition (unrestricted)

    • Original Schema 

    %YAML 1.1
---
$schema: "http://stsci.edu/schemas/yaml-schema/draft-01"
id: "http://stsci.edu/schemas/asdf/time/time-1.2.0"
title: Represents an instance in time.
description: |
  A "time" is a single instant in time.  It may explicitly specify the
  way time is represented (the "format") and the "scale" which
  specifies the offset and scaling relation of the unit of time.

  Specific emphasis is placed on supporting time scales (e.g. UTC,
  TAI, UT1, TDB) and time representations (e.g. JD, MJD, ISO 8601)
  that are used in astronomy and required to calculate, e.g., sidereal
  times and barycentric corrections.

  Times may be represented as one of the following:

  - an object, with explicit `value`, and optional `format`, `scale`
    and `location`.

  - a string, in which case the format is guessed from across
    the unambiguous options (`iso`, `byear`, `jyear`, `yday`), and the
    scale is hardcoded to `UTC`.

  In either case, a single time tag may be used to represent an
  n-dimensional array of times, using either an `ndarray` tag or
  inline as (possibly nested) YAML lists.  If YAML lists, the same
  format must be used for all time values.

  The precision of the numeric formats should only be assumed to be as
  good as an IEEE-754 double precision (float64) value.  If
  higher-precision is required, the `iso` or `yday` format should be
  used.

examples:
  -
    - Example ISO time
    - asdf-standard-1.6.0
    - |
        !time/time-1.2.0 "2000-12-31T13:05:27.737"

  -
    - Example year, day-of-year and time format time
    - asdf-standard-1.6.0
    - |
        !time/time-1.2.0 "2001:003:04:05:06.789"

  -
    - Example Besselian Epoch time
    - asdf-standard-1.6.0
    - |
        !time/time-1.2.0 B2000.0

  -
    - Example Besselian Epoch time, equivalent to above
    - asdf-standard-1.6.0
    - |
        !time/time-1.2.0
          value: 2000.0
          format: byear

  -
    - Example list of times
    - asdf-standard-1.6.0
    - |
        !time/time-1.2.0
          ["2000-12-31T13:05:27.737", "2000-12-31T13:06:38.444"]

  -
    - Example of an array of times
    - asdf-standard-1.6.0
    - |
        !time/time-1.2.0
          value: !core/ndarray-1.1.0
            data: [2000, 2001]
            datatype: float64
          format: jyear

  -
    - Example with a location
    - asdf-standard-1.6.0
    - |
        !time/time-1.2.0
          value: 2000.0
          format: jyear
          scale: tdb
          location:
            x: !unit/quantity-1.2.0
              value: 6378100
              unit: !unit/unit-1.0.0 m
            y: !unit/quantity-1.2.0
              value: 0
              unit: !unit/unit-1.0.0 m
            z: !unit/quantity-1.2.0
              value: 0
              unit: !unit/unit-1.0.0 m

definitions:
  iso_time:
    type: string
    pattern: "[0-9]{4}-(0[1-9])|(1[0-2])-(0[1-9])|([1-2][0-9])|(3[0-1])[T ]([0-1][0-9])|(2[0-4]):[0-5][0-9]:[0-5][0-9](.[0-9]+)?"

  byear:
    type: string
    pattern: "B[0-9]+(.[0-9]+)?"

  jyear:
    type: string
    pattern: "J[0-9]+(.[0-9]+)?"

  yday:
    type: string
    pattern: "[0-9]{4}:(00[1-9])|(0[1-9][0-9])|([1-2][0-9][0-9])|(3[0-5][0-9])|(36[0-5]):([0-1][0-9])|([0-1][0-9])|(2[0-4]):[0-5][0-9]:[0-5][0-9](.[0-9]+)?"

  string_formats:
    anyOf:
      - $ref: "#/definitions/iso_time"
      - $ref: "#/definitions/byear"
      - $ref: "#/definitions/jyear"
      - $ref: "#/definitions/yday"

  array_of_strings:
    type: array
    items:
      anyOf:
        - $ref: "#/definitions/array_of_strings"
        - $ref: "#/definitions/string_formats"

  format:
    description: |
      The format of the time.

      The supported formats are:

      - `iso`: ISO 8601 compliant date-time format
        `YYYY-MM-DDTHH:MM:SS.sss...`.  For example,
        `2000-01-01 00:00:00.000` is midnight on January 1,
        2000.  The `T` separating the date from the time
        section is optional.

      - `yday`: Year, day-of-year and time as
        `YYYY:DOY:HH:MM:SS.sss...`. The day-of-year (DOY) goes
        from 001 to 365 (366 in leap years). For example,
        `2000:001:00:00:00.000` is midnight on January 1,
        2000.

      - `byear`: Besselian Epoch year, eg. `B1950.0`.  The `B`
        is optional if the `byear` format is explicitly
        specified.

      - `jyear`: Julian Epoch year, eg. `J2000.0`.  The `J` is
        optional if the `jyear` format is explicitly
        specified.

      - `decimalyear`: Time as a decimal year, with integer
        values corresponding to midnight of the first day of
        each year. For example 2000.5 corresponds to the ISO
        time `2000-07-02 00:00:00`.

      - `jd`: Julian Date time format. This represents the
        number of days since the beginning of the Julian
        Period. For example, 2451544.5 in `jd` is midnight on
        January 1, 2000.

      - `mjd`: Modified Julian Date time format. This
        represents the number of days since midnight on
        November 17, 1858. For example, 51544.0 in MJD is
        midnight on January 1, 2000.

      - `gps`: GPS time: seconds from 1980-01-06 00:00:00 UTC
        For example, 630720013.0 is midnight on January 1,
        2000.

      - `unix`: Unix time: seconds from 1970-01-01 00:00:00
        UTC. For example, 946684800.0 in Unix time is midnight
        on January 1, 2000.  [TODO: Astropy's definition of
        UNIX time doesn't match POSIX's here.  What should we
        do for the purposes of ASDF?]

    enum:
      - byear
      - cxcsec
      - decimalyear
      - gps
      - iso
      - jd
      - jyear
      - mjd
      - unix
      - unix_tai
      - yday

  other_format:
    description: |
      The other formats supported by astropy.time:
      https://docs.astropy.org/en/latest/time/index.html#time-format

    enum:
      - byear_str
      - datetime
      - fits
      - isot
      - jyear_str
      - plot_date
      - ymdhms
      - datetime64

anyOf:
  - $ref: "#/definitions/string_formats"

  - $ref: "#/definitions/array_of_strings"

  - type: object
    properties:
      value:
        description: |
          The value(s) of the time.

        anyOf:
          - $ref: "#/definitions/string_formats"
          - $ref: "#/definitions/array_of_strings"
          - $ref: "../core/ndarray-1.1.0"
          - type: number

      format:
        description: |
          The format used to save the time in ASDF

          If not provided, the the format should be guessed from the
          string from among the following unambiguous options:
          `iso`, `byear`, `jyear` and `yday`.

        $ref: "#/definitions/format"

      base_format:
        description: |
          The original format of the time object

        oneOf:
          - $ref: "#/definitions/format"
          - $ref: "#/definitions/other_format"

      scale:
        description: |
          The time scale (or time standard) is a specification for
          measuring time: either the rate at which time passes; or
          points in time; or both. See also [3] and [4].

          These scales are defined in detail in [SOFA Time Scale and
          Calendar Tools](http://www.iausofa.org/sofa_ts_c.pdf).

          The supported time scales are:

          - `utc`: Coordinated Universal Time (UTC).  This is the
            default time scale, except for `gps`, `unix`.

          - `tai`: International Atomic Time (TAI).

          - `tcb`: Barycentric Coordinate Time (TCB).

          - `tcg`: Geocentric Coordinate Time (TCG).

          - `tdb`: Barycentric Dynamical Time (TDB).

          - `tt`: Terrestrial Time (TT).

          - `ut1`: Universal Time (UT1).

        enum:
          - utc
          - tai
          - tcb
          - tcg
          - tdb
          - tt
          - ut1

      location:
        description: |
          Specifies the observer location for scales that are
          sensitive to observer location, currently only `tdb`.  May
          be specified either with geocentric coordinates (X, Y, Z)
          with an optional unit or geodetic coordinates:
            - `long`: longitude in degrees
            - `lat`: in degrees
            - `h`: optional height

        type: object
        properties:
          x:
            $ref: "../unit/quantity-1.2.0"
          y:
            $ref: "../unit/quantity-1.2.0"
          z:
            $ref: "../unit/quantity-1.2.0"
        required: [x, y, z]

    required: [value]
...