diff --git a/.document b/.document
new file mode 100644
index 0000000000..439654c10a
--- /dev/null
+++ b/.document
@@ -0,0 +1,3 @@
+*.txt
+*.md
+lib/
diff --git a/.rdoc_options b/.rdoc_options
new file mode 100644
index 0000000000..f9297d90b5
--- /dev/null
+++ b/.rdoc_options
@@ -0,0 +1,2 @@
+---
+main_page: README.md
diff --git a/cmath.gemspec b/cmath.gemspec
index 8f2be11cbb..fa92bbc5a9 100644
--- a/cmath.gemspec
+++ b/cmath.gemspec
@@ -1,18 +1,19 @@
 # coding: utf-8
 # frozen_string_literal: true
 
+version = File.read("#{__dir__}/lib/cmath.rb")[/^\s*VERSION\s*=\s*"([^"]*)"/, 1]
 Gem::Specification.new do |spec|
   spec.name          = "cmath"
-  spec.version       = "1.0.0"
+  spec.version       = version
   spec.authors       = ["Tadayoshi Funaba"]
-  spec.email         = [nil]
+  spec.email         = ["tadf@dotrb.org"]
 
   spec.summary       = "Provides Trigonometric and Transcendental functions for complex numbers"
   spec.description   = "CMath is a library that provides trigonometric and transcendental functions for complex numbers. The functions in this module accept integers, floating-point numbers or complex numbers as arguments."
   spec.homepage      = "https://github.com/ruby/cmath"
   spec.license       = "BSD-2-Clause"
 
-  spec.files         = "lib/cmath.rb"
+  spec.files         = %w[.document .rdoc_options LICENSE.txt README.md lib/cmath.rb]
   spec.bindir        = "exe"
   spec.executables   = []
   spec.require_paths = ["lib"]
diff --git a/lib/cmath.rb b/lib/cmath.rb
index 0a301b718d..e4fe39e118 100644
--- a/lib/cmath.rb
+++ b/lib/cmath.rb
@@ -24,13 +24,20 @@ module CMath
 
   include Math
 
+  # The version string.
+  VERSION = "1.0.0"
+
   # Backup of Math is needed because mathn.rb replaces Math with CMath.
   RealMath = Math # :nodoc:
   private_constant :RealMath
 
   module_function
 
-  def domain?(z, nonreal = false)
+  # Returns +false+ if _z_ is real but not in domain by the block.
+  # If no block is given, domain is not limited.
+  # If _z_ is not real, returns _nonreal_ that is defaulted to +false+.
+  # If _z_ does not have +real?+ method, returns +true+.
+  private def domain?(z, nonreal = false) # :nodoc:
     unless z.respond_to?(:real?)
       return true
     end
diff --git a/rakelib/epoch.rake b/rakelib/epoch.rake
new file mode 100644
index 0000000000..80f27c9571
--- /dev/null
+++ b/rakelib/epoch.rake
@@ -0,0 +1,5 @@
+task "build" => "date_epoch"
+
+task "date_epoch" do
+  ENV["SOURCE_DATE_EPOCH"] = IO.popen(%W[git -C #{__dir__} log -1 --format=%ct], &:read).chomp
+end