diff --git a/lib/ougai/formatters/base.rb b/lib/ougai/formatters/base.rb
index 633f483..1672b8f 100644
--- a/lib/ougai/formatters/base.rb
+++ b/lib/ougai/formatters/base.rb
@@ -1,4 +1,3 @@
-require 'logger'
 require 'time'
 require 'socket'
 
@@ -13,6 +12,11 @@ def initialize(app_name = nil, hostname = nil)
         @hostname = hostname || Socket.gethostname.force_encoding('UTF-8')
         @trace_indent = 2
         @trace_max_lines = 100
+        self.datetime_format = nil
+      end
+
+      def datetime_format=(value)
+        @datetime_format = value || default_datetime_format
       end
 
       def serialize_exc(ex)
@@ -30,6 +34,19 @@ def serialize_trace(trace)
         sp = "\n" + ' ' * @trace_indent
         trace.slice(0, @trace_max_lines).join(sp)
       end
+
+      private
+
+      def format_datetime(time)
+        time.strftime(@datetime_format)
+      end
+
+      def default_datetime_format
+        t = Time.new
+        f = '%FT%T.%3N'
+        f << (t.utc? ? 'Z' : '%:z')
+        f.freeze
+      end
     end
   end
 end
diff --git a/lib/ougai/formatters/bunyan.rb b/lib/ougai/formatters/bunyan.rb
index 7661478..8de5422 100644
--- a/lib/ougai/formatters/bunyan.rb
+++ b/lib/ougai/formatters/bunyan.rb
@@ -49,7 +49,7 @@ def to_level(severity)
 
       def dump(data)
         return data unless @jsonize
-        data[:time] = data[:time].iso8601(3)
+        data[:time] = format_datetime(data[:time])
         str = JSON.generate(data)
         str << "\n" if @with_newline
         str
diff --git a/lib/ougai/formatters/readable.rb b/lib/ougai/formatters/readable.rb
index 4628ee6..2f99ed0 100644
--- a/lib/ougai/formatters/readable.rb
+++ b/lib/ougai/formatters/readable.rb
@@ -20,7 +20,7 @@ def initialize(opts = {})
       def call(severity, time, progname, data)
         msg = data.delete(:msg)
         level = @plain ? severity : colored_level(severity)
-        strs = ["[#{time.iso8601(3)}] #{level}: #{msg}"]
+        strs = ["[#{format_datetime(time)}] #{level}: #{msg}"]
         if err_str = create_err_str(data)
           strs.push(err_str)
         end
diff --git a/lib/ougai/version.rb b/lib/ougai/version.rb
index e5b4815..58d2d69 100644
--- a/lib/ougai/version.rb
+++ b/lib/ougai/version.rb
@@ -1,3 +1,3 @@
 module Ougai
-  VERSION = "1.5.1"
+  VERSION = "1.5.2"
 end
diff --git a/spec/formatters/base_spec.rb b/spec/formatters/base_spec.rb
index 3241f77..3685ae4 100644
--- a/spec/formatters/base_spec.rb
+++ b/spec/formatters/base_spec.rb
@@ -3,6 +3,24 @@
 describe Ougai::Formatters::Base do
   subject { described_class.new(app_name, hostname) }
 
+  context 'default' do
+    let (:app_name) { nil }
+    let (:hostname) { nil }
+
+    it 'has datetime format default ISO8601' do
+      expect(subject.datetime_format).to match(/^\%FT\%T\.\%3N(Z|\%\:z)$/)
+    end
+
+    it 'has datetime_format accessor' do
+      subject.datetime_format = '%I:%M:%S %p'
+      expect(subject.datetime_format).to eq('%I:%M:%S %p')
+
+      # revert default format by to set nil
+      subject.datetime_format = nil
+      expect(subject.datetime_format).to match(/^\%FT\%T\.\%3N(Z|\%\:z)$/)
+    end
+  end
+
   context 'without arguments and hostname contains a UTF-8 char' do
     let (:app_name) { nil }
     let (:hostname) { nil }
diff --git a/spec/formatters/bunyan_spec.rb b/spec/formatters/bunyan_spec.rb
index 81620c6..4c4e109 100644
--- a/spec/formatters/bunyan_spec.rb
+++ b/spec/formatters/bunyan_spec.rb
@@ -114,4 +114,20 @@
     end
   end
 
+  describe '#datetime_format' do
+    subject do
+      formatter.call('DEBUG', Time.now, nil, data)
+    end
+
+    context 'is time AM/PM format' do
+      before do
+        formatter.datetime_format = '%I:%M:%S %p'
+      end
+
+      it 'applys output' do
+        result = JSON.parse(subject, symbolize_names: true)
+        expect(result[:time]).to match(/^\d{2}:\d{2}:\d{2} [AP]M$/)
+      end
+    end
+  end
 end
diff --git a/spec/formatters/readable_spec.rb b/spec/formatters/readable_spec.rb
index b4d7696..ae15e81 100644
--- a/spec/formatters/readable_spec.rb
+++ b/spec/formatters/readable_spec.rb
@@ -1,6 +1,8 @@
 require 'spec_helper'
 
 describe Ougai::Formatters::Readable do
+  let!(:re_start_with_datetime) { /^\[\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}\.\d{3}(Z|[\+\-\:0-9]{4,6})]/ }
+
   let(:data) do
     {
       msg: 'Log Message!',
@@ -18,62 +20,73 @@
     }
   end
 
+  let(:formatter) { described_class.new }
+
   context 'when severity is TRACE' do
-    subject { described_class.new.call('TRACE', Time.now, nil, data) }
+    subject { formatter.call('TRACE', Time.now, nil, data) }
 
     it 'includes valid strings' do
+      expect(subject).to match(re_start_with_datetime)
       expect(subject).to include("\e[0;34mTRACE\e[0m: Log Message!")
       expect(subject.gsub(/\e\[([;\d]+)?m/, '')).to include(':status => 200')
     end
   end
 
   context 'when severity is DEBUG' do
-    subject { described_class.new.call('DEBUG', Time.now, nil, data) }
+    subject { formatter.call('DEBUG', Time.now, nil, data) }
 
     it 'includes valid strings' do
+      expect(subject).to match(re_start_with_datetime)
       expect(subject).to include("\e[0;37mDEBUG\e[0m: Log Message!")
       expect(subject.gsub(/\e\[([;\d]+)?m/, '')).to include(':status => 200')
     end
   end
 
   context 'when severity is INFO' do
-    subject { described_class.new.call('INFO', Time.now, nil, data) }
+    subject { formatter.call('INFO', Time.now, nil, data) }
 
     it 'includes valid strings' do
+      expect(subject).to match(re_start_with_datetime)
       expect(subject).to include("\e[0;36mINFO\e[0m: Log Message!")
       expect(subject.gsub(/\e\[([;\d]+)?m/, '')).to include(':method => "GET"')
     end
   end
 
   context 'when severity is WARN' do
-    subject { described_class.new.call('WARN', Time.now, nil, data) }
+    subject { formatter.call('WARN', Time.now, nil, data) }
 
     it 'includes valid strings' do
+      expect(subject).to match(re_start_with_datetime)
       expect(subject).to include("\e[0;33mWARN\e[0m: Log Message!")
       expect(subject.gsub(/\e\[([;\d]+)?m/, '')).to include(':path => "/"')
     end
   end
 
   context 'when severity is ERROR' do
-    subject { described_class.new.call('ERROR', Time.now, nil, data.merge({ err: err })) }
+    subject { formatter.call('ERROR', Time.now, nil, data.merge({ err: err })) }
 
     it 'includes valid strings' do
+      expect(subject).to match(re_start_with_datetime)
       expect(subject).to include("\e[0;31mERROR\e[0m: Log Message!")
       expect(subject.gsub(/\e\[([;\d]+)?m/, '')).to include('DummyError (it is dummy.):')
     end
   end
 
   context 'when severity is FATAL' do
-    subject { described_class.new.call('FATAL', Time.now, nil, { msg: 'TheEnd', err: err }) }
+    subject { formatter.call('FATAL', Time.now, nil, { msg: 'TheEnd', err: err }) }
+
     it 'includes valid strings' do
+      expect(subject).to match(re_start_with_datetime)
       expect(subject).to include("\e[0;35mFATAL\e[0m: TheEnd")
       expect(subject.gsub(/\e\[([;\d]+)?m/, '')).to include("error1.rb\n  error2.rb")
     end
   end
 
   context 'when severity is UNKNOWN' do
-    subject { described_class.new.call('ANY', Time.now, nil, { msg: 'unknown msg' }) }
+    subject { formatter.call('ANY', Time.now, nil, { msg: 'unknown msg' }) }
+
     it 'includes valid strings' do
+      expect(subject).to match(re_start_with_datetime)
       expect(subject).to include("\e[0;32mANY\e[0m: unknown msg")
     end
   end
@@ -91,4 +104,20 @@
       expect(plain_subject).not_to include(':method => "GET"')
     end
   end
+
+  describe '#datetime_format' do
+    subject do
+      formatter.call('DEBUG', Time.now, nil, data)
+    end
+
+    context 'is time AM/PM format' do
+      before do
+        formatter.datetime_format = '%I:%M:%S %p'
+      end
+
+      it 'applys output' do
+        expect(subject).to match(/^\[\d{2}:\d{2}:\d{2} [AP]M\]/)
+      end
+    end
+  end
 end