diff --git a/lib/fluent/time.rb b/lib/fluent/time.rb
index 8a4c12ac93..3fcddce259 100644
--- a/lib/fluent/time.rb
+++ b/lib/fluent/time.rb
@@ -363,6 +363,8 @@ def parse_float(value)
 
   class TimeFormatter
     def initialize(format = nil, localtime = true, timezone = nil)
+      # An empty cache slot is marked by a nil string, not by its key: 0 is a
+      # valid timestamp, so a key can never say whether the slot is filled.
       @tc1 = 0
       @tc1_str = nil
       @tc2 = 0
@@ -390,9 +392,9 @@ def initialize(format = nil, localtime = true, timezone = nil)
     end
 
     def format_without_subsec(time)
-      if @tc1 == time
+      if @tc1_str && @tc1 == time
         return @tc1_str
-      elsif @tc2 == time
+      elsif @tc2_str && @tc2 == time
         return @tc2_str
       else
         str = format_nocache(time)
@@ -408,9 +410,9 @@ def format_without_subsec(time)
     end
 
     def format_with_subsec(time)
-      if Fluent::EventTime.eq?(@tc1, time)
+      if @tc1_str && Fluent::EventTime.eq?(@tc1, time)
         return @tc1_str
-      elsif Fluent::EventTime.eq?(@tc2, time)
+      elsif @tc2_str && Fluent::EventTime.eq?(@tc2, time)
         return @tc2_str
       else
         str = format_nocache(time)
diff --git a/test/test_time_formatter.rb b/test/test_time_formatter.rb
index 0938e2fb5b..8db58fff58 100644
--- a/test/test_time_formatter.rb
+++ b/test/test_time_formatter.rb
@@ -196,6 +196,29 @@ def test_format_with_subsec
     assert_equal("20140927 0000.000000000", formatter.format(time))
   end
 
+  # The Unix epoch used to match an empty cache slot, whose string is nil.
+  def test_format_epoch
+    formatter = Fluent::TimeFormatter.new(nil, false, nil)
+    assert_equal("1970-01-01T00:00:00Z", formatter.format(Fluent::EventTime.new(0)))
+
+    formatter = Fluent::TimeFormatter.new(nil, false, nil)
+    assert_equal("1970-01-01T00:00:00Z", formatter.format(0))
+    assert_equal("1970-01-01T00:00:01Z", formatter.format(Fluent::EventTime.new(1)))
+    assert_equal("1970-01-01T00:00:00Z", formatter.format(Fluent::EventTime.new(0)))
+  end
+
+  # Here the slot is compared with EventTime.eq?, which falls back to comparing
+  # seconds alone, so the whole first second of the epoch matched an empty slot.
+  def test_format_epoch_with_subsec
+    formatter = Fluent::TimeFormatter.new("%Y%m%d %H%M%S.%N", false, nil)
+    assert_equal("19700101 000000.000000000", formatter.format(Fluent::EventTime.new(0, 0)))
+    assert_equal("19700101 000000.123456789", formatter.format(Fluent::EventTime.new(0, 123456789)))
+    assert_equal("19700101 000000.000000000", formatter.format(Fluent::EventTime.new(0, 0)))
+
+    formatter = Fluent::TimeFormatter.new("%Y%m%d %H%M%S.%N", false, nil)
+    assert_equal("19700101 000000.999999999", formatter.format(Fluent::EventTime.new(0, 999999999)))
+  end
+
   sub_test_case 'TimeMixin::Formatter' do
     class DummyForTimeFormatter
       include Fluent::Configurable