add a utility submodule to define 'TZ' on Windows

lib/jekyll.rb

@@ -119,7 +119,7 @@ module Jekyll
     # Returns nothing
     # rubocop:disable Style/AccessorMethodName
     def set_timezone(timezone)
-      ENV["TZ"] = timezone
+      ENV["TZ"] = Utils::Platforms.windows? ? Utils::WinTZ.calculate(timezone) : timezone
     end
     # rubocop:enable Style/AccessorMethodName
 

lib/jekyll/utils.rb

@@ -4,6 +4,7 @@ module Jekyll
     extend self
     autoload :Platforms, "jekyll/utils/platforms"
     autoload :Ansi, "jekyll/utils/ansi"
+    autoload :WinTZ, "jekyll/utils/win_tz"
 
     # Constants for use in #slugify
     SLUGIFY_MODES = %w(raw default pretty ascii).freeze

lib/jekyll/utils/win_tz.rb

@@ -0,0 +1,73 @@
+module Jekyll
+  module Utils
+    module WinTZ
+      extend self
+
+      # Public: Calculate the Timezone for Windows when the config file has a defined
+      #         'timezone' key.
+      #
+      # timezone - the IANA Time Zone specified in "_config.yml"
+      #
+      # Returns a string that ultimately re-defines ENV["TZ"] in Windows
+      def calculate(timezone)
+        External.require_with_graceful_fail("tzinfo")
+        tz = TZInfo::Timezone.get(timezone)
+        difference = Time.now.to_i - tz.now.to_i
+        #
+        # POSIX style definition reverses the offset sign.
+        #   e.g. Eastern Standard Time (EST) that is 5Hrs. to the 'west' of Prime Meridian
+        #   is denoted as:
+        #     EST+5 (or) EST+05:00
+        # Reference: http://www.gnu.org/software/libc/manual/html_node/TZ-Variable.html
+        sign = difference < 0 ? "-" : "+"
+        offset = sign == "-" ? "+" : "-" unless difference.zero?
+        #
+        # convert the difference (in seconds) to hours, as a rational number, and perform
+        # a modulo operation on it.
+        modulo = modulo_of(rational_hour(difference))
+        #
+        # Format the hour as a two-digit number.
+        # Establish the minutes based on modulo expression.
+        hh = format("%02d", absolute_hour(difference).ceil)
+        mm = modulo.zero? ? "00" : "30"
+
+        Jekyll.logger.debug "Timezone:", "#{timezone} #{offset}#{hh}:#{mm}"
+        #
+        # Note: The 3-letter-word below doesn't have a particular significance.
+        "WTZ#{sign}#{hh}:#{mm}"
+      end
+
+      private
+
+      # Private: Convert given seconds to an hour as a rational number.
+      #
+      # seconds - supplied as an integer, it is converted to a rational number.
+      # 3600 - no. of seconds in an hour.
+      #
+      # Returns a rational number.
+      def rational_hour(seconds)
+        seconds.to_r/3600
+      end
+
+      # Private: Convert given seconds to an hour as an absolute number.
+      #
+      # seconds - supplied as an integer, it is converted to its absolute.
+      # 3600 - no. of seconds in an hour.
+      #
+      # Returns an integer.
+      def absolute_hour(seconds)
+        seconds.abs/3600
+      end
+
+      # Private: Perform a modulo operation on a given fraction.
+      #
+      # fraction - supplied as a rational number, its numerator is divided
+      #            by its denominator and the remainder returned.
+      #
+      # Returns an integer.
+      def modulo_of(fraction)
+        fraction.numerator % fraction.denominator
+      end
+    end
+  end
+end