Build instants from a Julian Date read in a scale

[rhannequin]

This replaces from_tai_julian_date with from_julian_date, which reads the Julian Date in a scale and stores the instant in TAI. An instant given in one scale can then be read back in another.

instant = Horologium::Instant.from_julian_date(2_443_144.5, scale: :tai)

instant.to(:tt).as(:julian_date)
# => 2443144.5003725

The Julian Date can be given in several shapes. A String and a Rational give it exactly, a high and a low Float give it to about twice what one Float holds, and a single Float is worth about 40 microseconds at a modern date. A String must be written as digits with an optional decimal fraction and anything else raises the new ParseError.

Horologium::Instant.from_julian_date("2456463.052272", scale: :tt)
Horologium::Instant.from_julian_date(2_456_463.0, 0.052272, scale: :tt)

from_modified_julian_date builds an instant from the Modified Julian Date, the Julian Date counted from a later origin. It takes the same shapes. Representations::ModifiedJulianDate reads an instant back in it.

instant = Horologium::Instant.from_modified_julian_date(60_796.0, scale: :tai)

instant.as(:modified_julian_date, scale: :tai)
# => 60796.0

[Copilot]

Pull request overview

This PR updates Horologium’s Instant construction APIs to accept Julian Date inputs expressed in an explicit scale (while storing internally in TAI), adds strict parsing for string inputs (with a new ParseError), and introduces Modified Julian Date support as both an input and an output representation.

Changes:

• Replace Instant.from_tai_julian_date usage with Instant.from_julian_date(..., scale:), enabling cross-scale round-tripping.
• Add strict Julian Date parsing for String/Rational/two-part floats, raising Horologium::ParseError for invalid strings.
• Introduce Modified Julian Date (from_modified_julian_date + Representations::ModifiedJulianDate) and expose it via ScaleReading#as.

Reviewed changes

Copilot reviewed 21 out of 21 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
test/test_scale_reading.rb	Updates scale reading tests to use from_julian_date and adds MJD readout coverage.
test/test_instant.rb	Expands Instant construction tests for multi-shape Julian Date inputs, strict parsing, and MJD support.
test/test_error.rb	Adds coverage that ParseError descends from Horologium::Error.
test/test_configuration.rb	Updates configuration tests to build instants via from_julian_date(..., scale: :tai).
test/scales/test_tai.rb	Updates TAI scale tests to use the new constructor.
test/representations/test_modified_julian_date.rb	Adds new test suite for Modified Julian Date rendering/parsing.
test/representations/test_julian_date.rb	Adds parsing tests for new Julian Date input shapes and error cases.
sig/horologium/scale_reading.rbs	Extends representation map typing to include ModifiedJulianDate.
sig/horologium/representations/modified_julian_date.rbs	Adds RBS signatures for the new MJD representation.
sig/horologium/representations/julian_date.rbs	Extends RBS to include parsing API and helpers (needs type tightening for private helpers).
sig/horologium/instant.rbs	Replaces old constructor signature with from_julian_date / from_modified_julian_date and shared types.
sig/horologium.rbs	Adds ParseError to the public type surface.
README.md	Updates public docs/examples to the new constructors and adds MJD examples + input-shape guidance.
lib/horologium/scales/tt.rb	Updates TT docs/examples to use from_julian_date(..., scale:).
lib/horologium/scales/tai.rb	Updates TAI docs/examples to use from_julian_date(..., scale:).
lib/horologium/scale_reading.rb	Adds :modified_julian_date as a supported ScaleReading#as representation.
lib/horologium/representations/modified_julian_date.rb	Implements Modified Julian Date render/parse and precision-specific offsets.
lib/horologium/representations/julian_date.rb	Adds strict string parsing + multi-shape parsing entrypoint and shared render_value.
lib/horologium/instant.rb	Introduces scale-aware from_julian_date and from_modified_julian_date constructors via a shared helper.
lib/horologium/error.rb	Adds ParseError definition and documentation.
lib/horologium.rb	Requires the new Modified Julian Date representation.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.