diff --git a/src/util.cc b/src/util.cc
index b40fd40c..635439fb 100644
--- a/src/util.cc
+++ b/src/util.cc
@@ -167,7 +167,7 @@ std::string quote_string(const std::string& target)
 
 namespace {
 template<typename Iterator>
-Iterator cpydig(Iterator d, int n, size_t len)
+Iterator cpydig(Iterator d, uint32_t n, size_t len)
 {
   auto p = d + len - 1;
 
@@ -223,6 +223,100 @@ std::string http_date(time_t t)
   return res;
 }
 
+std::string common_log_date(time_t t)
+{
+  struct tm tms;
+
+  if(localtime_r(&t, &tms) == nullptr) {
+    return "";
+  }
+
+  // Format data like this:
+  // 03/Jul/2014:00:19:38 +0900
+  std::string res;
+  res.resize(26);
+
+  auto p = std::begin(res);
+
+  p = cpydig(p, tms.tm_mday, 2);
+  *p++ = '/';
+  auto s = MONTH[tms.tm_mon];
+  p = std::copy(s, s + 3, p);
+  *p++ = '/';
+  p = cpydig(p, tms.tm_year + 1900, 4);
+  *p++ = ':';
+  p = cpydig(p, tms.tm_hour, 2);
+  *p++ = ':';
+  p = cpydig(p, tms.tm_min, 2);
+  *p++ = ':';
+  p = cpydig(p, tms.tm_sec, 2);
+  *p++ = ' ';
+
+  auto gmtoff = tms.tm_gmtoff;
+  if(gmtoff >= 0) {
+    *p++ = '+';
+  } else {
+    *p++ = '-';
+    gmtoff = -gmtoff;
+  }
+
+  p = cpydig(p, gmtoff / 3600, 2);
+  p = cpydig(p, (gmtoff % 3600) / 60, 2);
+
+  return res;
+}
+
+std::string iso8601_date(int64_t ms)
+{
+  time_t sec = ms / 1000;
+
+  tm tms;
+  if(localtime_r(&sec, &tms) == nullptr) {
+    return "";
+  }
+
+  // Format data like this:
+  // 2014-11-15T12:58:24.741Z
+  // 2014-11-15T12:58:24.741+09:00
+  std::string res;
+  res.resize(29);
+
+  auto p = std::begin(res);
+
+  p = cpydig(p, tms.tm_year + 1900, 4);
+  *p++ = '-';
+  p = cpydig(p, tms.tm_mon + 1, 2);
+  *p++ = '-';
+  p = cpydig(p, tms.tm_mday, 2);
+  *p++ = 'T';
+  p = cpydig(p, tms.tm_hour, 2);
+  *p++ = ':';
+  p = cpydig(p, tms.tm_min, 2);
+  *p++ = ':';
+  p = cpydig(p, tms.tm_sec, 2);
+  *p++ = '.';
+  p = cpydig(p, ms % 1000, 3);
+
+  auto gmtoff = tms.tm_gmtoff;
+  if(gmtoff == 0) {
+    *p++ = 'Z';
+  } else {
+    if(gmtoff > 0) {
+      *p++ = '+';
+    } else {
+      *p++ = '-';
+      gmtoff = -gmtoff;
+    }
+    p = cpydig(p, gmtoff / 3600, 2);
+    *p++ = ':';
+    p = cpydig(p, (gmtoff % 3600) / 60, 2);
+  }
+
+  res.resize(p - std::begin(res));
+
+  return res;
+}
+
 time_t parse_http_date(const std::string& s)
 {
   tm tm;
diff --git a/src/util.h b/src/util.h
index 94f51f94..f0ddced0 100644
--- a/src/util.h
+++ b/src/util.h
@@ -212,6 +212,14 @@ std::string format_hex(const unsigned char *s, size_t len);
 
 std::string http_date(time_t t);
 
+// Returns given time |t| from epoch in Common Log format (e.g.,
+// 03/Jul/2014:00:19:38 +0900)
+std::string common_log_date(time_t t);
+
+// Returns given millisecond |ms| from epoch in ISO 8601 format (e.g.,
+// 2014-11-15T12:58:24.741Z)
+std::string iso8601_date(int64_t ms);
+
 time_t parse_http_date(const std::string& s);
 
 template<typename InputIterator1, typename InputIterator2>
@@ -489,21 +497,11 @@ std::vector<unsigned char> get_default_alpn();
 template <typename T>
 std::string format_common_log(const T& tp)
 {
-  auto t = std::chrono::duration_cast<std::chrono::milliseconds>
+  auto t = std::chrono::duration_cast<std::chrono::seconds>
     (tp.time_since_epoch());
-  time_t sec = t.count() / 1000;
-
-  tm tms;
-  if(localtime_r(&sec, &tms) == nullptr) {
-    return "";
-  }
-
-  char buf[32];
-
-  strftime(buf, sizeof(buf), "%d/%b/%Y:%T %z", &tms);
-
-  return buf;
+  return common_log_date(t.count());
 }
+
 // Returns given time |tp| in ISO 8601 format (e.g.,
 // 2014-11-15T12:58:24.741Z)
 // Expected type of |tp| is std::chrono::timepoint
@@ -512,20 +510,7 @@ std::string format_iso8601(const T& tp)
 {
   auto t = std::chrono::duration_cast<std::chrono::milliseconds>
     (tp.time_since_epoch());
-  time_t sec = t.count() / 1000;
-
-  tm tms;
-  if(gmtime_r(&sec, &tms) == nullptr) {
-    return "";
-  }
-
-  char buf[128];
-
-  auto nwrite = strftime(buf, sizeof(buf), "%Y-%m-%dT%H:%M:%S", &tms);
-  snprintf(&buf[nwrite], sizeof(buf) - nwrite, ".%03ldZ",
-      static_cast<int>(t.count() % 1000));
-
-  return buf;
+  return iso8601_date(t.count());
 }
 
 // Return the system precision of the template parameter |Clock| as