Add decimal_hour_to_time util (#53)

Sometimes, the time of the day is available as a decimal number instead
of the three usual components hours/minutes/seconds.

This new util method enables to convert a time as a decimal number into
an actual `Time` object (in UTC).

lib/astronoby.rb

@@ -21,6 +21,7 @@
 require "astronoby/time/greenwich_sidereal_time"
 require "astronoby/time/local_sidereal_time"
 require "astronoby/util/astrodynamics"
+require "astronoby/util/time"
 require "astronoby/util/trigonometry"
 require "astronoby/true_obliquity"
 require "astronoby/version"

lib/astronoby/util/time.rb

@@ -0,0 +1,26 @@
+# frozen_string_literal: true
+
+module Astronoby
+  module Util
+    module Time
+      # @param date [Date]
+      # @param decimal [Numeric] Hour of the day, in decimal hours
+      # @return [::Time] Date and time
+      def self.decimal_hour_to_time(date, decimal)
+        absolute_hour = decimal.abs
+        hour = absolute_hour.floor
+
+        unless hour.between?(0, 24)
+          raise IncompatibleArgumentsError, "Hour must be between 0 and 24, got #{hour}"
+        end
+
+        decimal_minute = 60 * (absolute_hour - hour)
+        absolute_decimal_minute = (60 * (absolute_hour - hour)).abs
+        minute = decimal_minute.floor
+        second = 60 * (absolute_decimal_minute - absolute_decimal_minute.floor)
+
+        ::Time.utc(date.year, date.month, date.day, hour, minute, second).round
+      end
+    end
+  end
+end

spec/astronoby/util/time_spec.rb

@@ -0,0 +1,31 @@
+# frozen_string_literal: true
+
+RSpec.describe Astronoby::Util::Time do
+  describe "::decimal_hour_to_time" do
+    it "returns a Time object" do
+      date = Date.new
+      decimal = 0
+
+      expect(described_class.decimal_hour_to_time(date, decimal)).to be_a Time
+    end
+
+    it "converts a decimal time to a Time format" do
+      date = Date.new(2024, 3, 14)
+      decimal = 12.34
+
+      time = described_class.decimal_hour_to_time(date, decimal)
+
+      expect(time).to eq Time.utc(2024, 3, 14, 12, 20, 24)
+    end
+
+    context "when the decimal time is out of range" do
+      it "raises an error" do
+        expect { described_class.decimal_hour_to_time(Date.new, 25) }
+          .to raise_error(
+            Astronoby::IncompatibleArgumentsError,
+            "Hour must be between 0 and 24, got 25"
+          )
+      end
+    end
+  end
+end