diff --git a/doc/nghttpx.h2r b/doc/nghttpx.h2r
index 384127ee..cad39b16 100644
--- a/doc/nghttpx.h2r
+++ b/doc/nghttpx.h2r
@@ -388,6 +388,16 @@ respectively.
 
         Return the serial number of a client certificate.
 
+    .. rb:attr_reader:: tls_client_not_before
+
+        Return the start date of a client certificate in seconds since
+        the epoch.
+
+    .. rb:attr_reader:: tls_client_not_after
+
+        Return the end date of a client certificate in seconds since
+        the epoch.
+
     .. rb:attr_reader:: tls_cipher
 
         Return a TLS cipher negotiated in this connection.
diff --git a/src/shrpx_mruby_module_env.cc b/src/shrpx_mruby_module_env.cc
index 70cdcea7..9a7c3640 100644
--- a/src/shrpx_mruby_module_env.cc
+++ b/src/shrpx_mruby_module_env.cc
@@ -259,6 +259,58 @@ mrb_value env_get_tls_client_serial(mrb_state *mrb, mrb_value self) {
 }
 } // namespace
 
+namespace {
+mrb_value env_get_tls_client_not_before(mrb_state *mrb, mrb_value self) {
+  auto data = static_cast<MRubyAssocData *>(mrb->ud);
+  auto downstream = data->downstream;
+  auto upstream = downstream->get_upstream();
+  auto handler = upstream->get_client_handler();
+  auto ssl = handler->get_ssl();
+
+  if (!ssl) {
+    return mrb_fixnum_value(0);
+  }
+
+  auto x = SSL_get_peer_certificate(ssl);
+  if (!x) {
+    return mrb_fixnum_value(0);
+  }
+
+  time_t t;
+  if (tls::get_x509_not_before(t, x) != 0) {
+    return mrb_fixnum_value(0);
+  }
+
+  return mrb_fixnum_value(t);
+}
+} // namespace
+
+namespace {
+mrb_value env_get_tls_client_not_after(mrb_state *mrb, mrb_value self) {
+  auto data = static_cast<MRubyAssocData *>(mrb->ud);
+  auto downstream = data->downstream;
+  auto upstream = downstream->get_upstream();
+  auto handler = upstream->get_client_handler();
+  auto ssl = handler->get_ssl();
+
+  if (!ssl) {
+    return mrb_fixnum_value(0);
+  }
+
+  auto x = SSL_get_peer_certificate(ssl);
+  if (!x) {
+    return mrb_fixnum_value(0);
+  }
+
+  time_t t;
+  if (tls::get_x509_not_after(t, x) != 0) {
+    return mrb_fixnum_value(0);
+  }
+
+  return mrb_fixnum_value(t);
+}
+} // namespace
+
 namespace {
 mrb_value env_get_tls_cipher(mrb_state *mrb, mrb_value self) {
   auto data = static_cast<MRubyAssocData *>(mrb->ud);
@@ -374,6 +426,10 @@ void init_env_class(mrb_state *mrb, RClass *module) {
                     env_get_tls_client_subject_name, MRB_ARGS_NONE());
   mrb_define_method(mrb, env_class, "tls_client_serial",
                     env_get_tls_client_serial, MRB_ARGS_NONE());
+  mrb_define_method(mrb, env_class, "tls_client_not_before",
+                    env_get_tls_client_not_before, MRB_ARGS_NONE());
+  mrb_define_method(mrb, env_class, "tls_client_not_after",
+                    env_get_tls_client_not_after, MRB_ARGS_NONE());
   mrb_define_method(mrb, env_class, "tls_cipher", env_get_tls_cipher,
                     MRB_ARGS_NONE());
   mrb_define_method(mrb, env_class, "tls_protocol", env_get_tls_protocol,
diff --git a/src/shrpx_tls.cc b/src/shrpx_tls.cc
index d0bf6085..6d07e2cf 100644
--- a/src/shrpx_tls.cc
+++ b/src/shrpx_tls.cc
@@ -64,6 +64,7 @@
 #include "tls.h"
 #include "template.h"
 #include "ssl_compat.h"
+#include "timegm.h"
 
 using namespace nghttp2;
 
@@ -1992,6 +1993,74 @@ StringRef get_x509_serial(BlockAllocator &balloc, X509 *x) {
 #endif // !OPENSSL_1_1_API
 }
 
+namespace {
+// Performs conversion from |at| to time_t.  The result is stored in
+// |t|.  This function returns 0 if it succeeds, or -1.
+int time_t_from_asn1_time(time_t &t, const ASN1_TIME *at) {
+  int rv;
+
+#if OPENSSL_1_1_1_API
+  struct tm tm;
+  rv = ASN1_TIME_to_tm(at, &tm);
+  if (rv != 1) {
+    return -1;
+  }
+
+  t = nghttp2_timegm(&tm);
+#else  // !OPENSSL_1_1_1_API
+  auto b = BIO_new(BIO_s_mem());
+  if (!b) {
+    return -1;
+  }
+
+  auto bio_deleter = defer(BIO_free, b);
+
+  rv = ASN1_TIME_print(b, at);
+  if (rv != 1) {
+    return -1;
+  }
+
+  unsigned char *s;
+  auto slen = BIO_get_mem_data(b, &s);
+  auto tt = util::parse_openssl_asn1_time_print(
+      StringRef{s, static_cast<size_t>(slen)});
+  if (tt == 0) {
+    return -1;
+  }
+
+  t = tt;
+#endif // !OPENSSL_1_1_1_API
+
+  return 0;
+}
+} // namespace
+
+int get_x509_not_before(time_t &t, X509 *x) {
+#if OPENSSL_1_1_API
+  auto at = X509_get0_notBefore(x);
+#else  // !OPENSSL_1_1_API
+  auto at = X509_get_notBefore(x);
+#endif // !OPENSSL_1_1_API
+  if (!at) {
+    return -1;
+  }
+
+  return time_t_from_asn1_time(t, at);
+}
+
+int get_x509_not_after(time_t &t, X509 *x) {
+#if OPENSSL_1_1_API
+  auto at = X509_get0_notAfter(x);
+#else  // !OPENSSL_1_1_API
+  auto at = X509_get_notAfter(x);
+#endif // !OPENSSL_1_1_API
+  if (!at) {
+    return -1;
+  }
+
+  return time_t_from_asn1_time(t, at);
+}
+
 } // namespace tls
 
 } // namespace shrpx
diff --git a/src/shrpx_tls.h b/src/shrpx_tls.h
index a368dabf..f3ee63c2 100644
--- a/src/shrpx_tls.h
+++ b/src/shrpx_tls.h
@@ -288,6 +288,14 @@ StringRef get_x509_issuer_name(BlockAllocator &balloc, X509 *x);
 // number, it returns an empty string.  number
 StringRef get_x509_serial(BlockAllocator &balloc, X509 *x);
 
+// Fills NotBefore of |x| in |t|.  This function returns 0 if it
+// succeeds, or -1.
+int get_x509_not_before(time_t &t, X509 *x);
+
+// Fills NotAfter of |x| in |t|.  This function returns 0 if it
+// succeeds, or -1.
+int get_x509_not_after(time_t &t, X509 *x);
+
 } // namespace tls
 
 } // namespace shrpx
diff --git a/src/util.cc b/src/util.cc
index 606d1b15..36ca5b2e 100644
--- a/src/util.cc
+++ b/src/util.cc
@@ -407,6 +407,15 @@ time_t parse_http_date(const StringRef &s) {
 #endif // !_WIN32
 }
 
+time_t parse_openssl_asn1_time_print(const StringRef &s) {
+  tm tm{};
+  auto r = strptime(s.c_str(), "%b %d %H:%M:%S %Y GMT", &tm);
+  if (r == nullptr) {
+    return 0;
+  }
+  return nghttp2_timegm_without_yday(&tm);
+}
+
 char upcase(char c) {
   if ('a' <= c && c <= 'z') {
     return c - 'a' + 'A';
diff --git a/src/util.h b/src/util.h
index 66f83de7..cc85b2c5 100644
--- a/src/util.h
+++ b/src/util.h
@@ -196,6 +196,11 @@ char *iso8601_date(char *res, int64_t ms);
 
 time_t parse_http_date(const StringRef &s);
 
+// Parses time formatted as "MMM DD HH:MM:SS YYYY [GMT]" (e.g., Feb 3
+// 00:55:52 2015 GMT), which is specifically used by OpenSSL
+// ASN1_TIME_print().
+time_t parse_openssl_asn1_time_print(const StringRef &s);
+
 char upcase(char c);
 
 inline char lowcase(char c) {