diff --git a/ChangeLog b/ChangeLog
index 37cf750..820c26b 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,19 @@
 mbed TLS ChangeLog (Sorted per branch, date)
 
+= mbed TLS 2.13.1 branch released 2018-09-06
+
+API Changes
+   * Extend the platform module with an abstraction mbedtls_platform_gmtime_r()
+     whose implementation should behave as a thread-safe version of gmtime().
+     This allows users to configure such an implementation at compile time when
+     the target system cannot be deduced automatically, by setting the option
+     MBEDTLS_PLATFORM_GMTIME_R_ALT. At this stage Mbed TLS is only able to
+     automatically select implementations for Windows and POSIX C libraries.
+
+Bugfix
+   * Fix build failures on platforms where only gmtime() is available but
+     neither gmtime_r() nor gmtime_s() are present. Fixes #1907.
+
 = mbed TLS 2.13.0 branch released 2018-08-31
 
 Security
diff --git a/include/mbedtls/config.h b/include/mbedtls/config.h
index 052aed0..81438c5 100644
--- a/include/mbedtls/config.h
+++ b/include/mbedtls/config.h
@@ -137,12 +137,21 @@
 /**
  * \def MBEDTLS_HAVE_TIME_DATE
  *
- * System has time.h and time(), gmtime() and the clock is correct.
+ * System has time.h, time(), and an implementation for
+ * mbedtls_platform_gmtime_r() (see below).
  * The time needs to be correct (not necesarily very accurate, but at least
  * the date should be correct). This is used to verify the validity period of
  * X.509 certificates.
  *
  * Comment if your system does not have a correct clock.
+ *
+ * \note mbedtls_platform_gmtime_r() is an abstraction in platform_util.h that
+ * behaves similarly to the gmtime_r() function from the C standard. Refer to
+ * the documentation for mbedtls_platform_gmtime_r() for more information.
+ *
+ * \note It is possible to configure an implementation for
+ * mbedtls_platform_gmtime_r() at compile-time by using the macro
+ * MBEDTLS_PLATFORM_GMTIME_R_ALT.
  */
 #define MBEDTLS_HAVE_TIME_DATE
 
@@ -3100,6 +3109,25 @@
  */
 //#define MBEDTLS_PLATFORM_ZEROIZE_ALT
 
+/**
+ * Uncomment the macro to let Mbed TLS use your alternate implementation of
+ * mbedtls_platform_gmtime_r(). This replaces the default implementation in
+ * platform_util.c.
+ *
+ * gmtime() is not a thread-safe function as defined in the C standard. The
+ * library will try to use safer implementations of this function, such as
+ * gmtime_r() when available. However, if Mbed TLS cannot identify the target
+ * system, the implementation of mbedtls_platform_gmtime_r() will default to
+ * using the standard gmtime(). In this case, calls from the library to
+ * gmtime() will be guarded by the global mutex mbedtls_threading_gmtime_mutex
+ * if MBEDTLS_THREADING_C is enabled. We recommend that calls from outside the
+ * library are also guarded with this mutex to avoid race conditions. However,
+ * if the macro MBEDTLS_PLATFORM_GMTIME_R_ALT is defined, Mbed TLS will
+ * unconditionally use the implementation for mbedtls_platform_gmtime_r()
+ * supplied at compile time.
+ */
+//#define MBEDTLS_PLATFORM_GMTIME_R_ALT
+
 /* \} name SECTION: Customisation configuration options */
 
 /* Target and application specific configurations */
diff --git a/include/mbedtls/platform_util.h b/include/mbedtls/platform_util.h
index 84f0732..164a1a0 100644
--- a/include/mbedtls/platform_util.h
+++ b/include/mbedtls/platform_util.h
@@ -25,7 +25,17 @@
 #ifndef MBEDTLS_PLATFORM_UTIL_H
 #define MBEDTLS_PLATFORM_UTIL_H
 
+#if !defined(MBEDTLS_CONFIG_FILE)
+#include "mbedtls/config.h"
+#else
+#include MBEDTLS_CONFIG_FILE
+#endif
+
 #include <stddef.h>
+#if defined(MBEDTLS_HAVE_TIME_DATE)
+#include "mbedtls/platform_time.h"
+#include <time.h>
+#endif /* MBEDTLS_HAVE_TIME_DATE */
 
 #ifdef __cplusplus
 extern "C" {
@@ -55,6 +65,37 @@
  */
 void mbedtls_platform_zeroize( void *buf, size_t len );
 
+#if defined(MBEDTLS_HAVE_TIME_DATE)
+/**
+ * \brief      Platform-specific implementation of gmtime_r()
+ *
+ *             The function is a thread-safe abstraction that behaves
+ *             similarly to the gmtime_r() function from Unix/POSIX.
+ *
+ *             Mbed TLS will try to identify the underlying platform and
+ *             make use of an appropriate underlying implementation (e.g.
+ *             gmtime_r() for POSIX and gmtime_s() for Windows). If this is
+ *             not possible, then gmtime() will be used. In this case, calls
+ *             from the library to gmtime() will be guarded by the mutex
+ *             mbedtls_threading_gmtime_mutex if MBEDTLS_THREADING_C is
+ *             enabled. It is recommended that calls from outside the library
+ *             are also guarded by this mutex.
+ *
+ *             If MBEDTLS_PLATFORM_GMTIME_R_ALT is defined, then Mbed TLS will
+ *             unconditionally use the alternative implementation for
+ *             mbedtls_platform_gmtime_r() supplied by the user at compile time.
+ *
+ * \param tt     Pointer to an object containing time (in seconds) since the
+ *               epoch to be converted
+ * \param tm_buf Pointer to an object where the results will be stored
+ *
+ * \return      Pointer to an object of type struct tm on success, otherwise
+ *              NULL
+ */
+struct tm *mbedtls_platform_gmtime_r( const mbedtls_time_t *tt,
+                                      struct tm *tm_buf );
+#endif /* MBEDTLS_HAVE_TIME_DATE */
+
 #ifdef __cplusplus
 }
 #endif
diff --git a/include/mbedtls/threading.h b/include/mbedtls/threading.h
index 5112ebb..75298bf 100644
--- a/include/mbedtls/threading.h
+++ b/include/mbedtls/threading.h
@@ -99,6 +99,17 @@
 #if defined(MBEDTLS_FS_IO)
 extern mbedtls_threading_mutex_t mbedtls_threading_readdir_mutex;
 #endif
+
+#if defined(MBEDTLS_HAVE_TIME_DATE) && !defined(MBEDTLS_PLATFORM_GMTIME_R_ALT)
+/* This mutex may or may not be used in the default definition of
+ * mbedtls_platform_gmtime_r(), but in order to determine that,
+ * we need to check POSIX features, hence modify _POSIX_C_SOURCE.
+ * With the current approach, this declaration is orphaned, lacking
+ * an accompanying definition, in case mbedtls_platform_gmtime_r()
+ * doesn't need it, but that's not a problem. */
+extern mbedtls_threading_mutex_t mbedtls_threading_gmtime_mutex;
+#endif /* MBEDTLS_HAVE_TIME_DATE && !MBEDTLS_PLATFORM_GMTIME_R_ALT */
+
 #endif /* MBEDTLS_THREADING_C */
 
 #ifdef __cplusplus
diff --git a/library/platform_util.c b/library/platform_util.c
index 1a57de9..ca5fe4f 100644
--- a/library/platform_util.c
+++ b/library/platform_util.c
@@ -20,6 +20,14 @@
  *  This file is part of Mbed TLS (https://tls.mbed.org)
  */
 
+/*
+ * Ensure gmtime_r is available even with -std=c99; must be defined before
+ * config.h, which pulls in glibc's features.h. Harmless on other platforms.
+ */
+#if !defined(_POSIX_C_SOURCE)
+#define _POSIX_C_SOURCE 200112L
+#endif
+
 #if !defined(MBEDTLS_CONFIG_FILE)
 #include "mbedtls/config.h"
 #else
@@ -27,6 +35,7 @@
 #endif
 
 #include "mbedtls/platform_util.h"
+#include "mbedtls/threading.h"
 
 #include <stddef.h>
 #include <string.h>
@@ -65,3 +74,62 @@
     memset_func( buf, 0, len );
 }
 #endif /* MBEDTLS_PLATFORM_ZEROIZE_ALT */
+
+#if defined(MBEDTLS_HAVE_TIME_DATE) && !defined(MBEDTLS_PLATFORM_GMTIME_R_ALT)
+#include <time.h>
+#if !defined(_WIN32) && (defined(unix) || \
+    defined(__unix) || defined(__unix__) || (defined(__APPLE__) && \
+    defined(__MACH__)))
+#include <unistd.h>
+#endif /* !_WIN32 && (unix || __unix || __unix__ ||
+        * (__APPLE__ && __MACH__)) */
+
+#if !( ( defined(_POSIX_VERSION) && _POSIX_VERSION >= 200809L ) ||     \
+       ( defined(_POSIX_THREAD_SAFE_FUNCTIONS ) &&                     \
+         _POSIX_THREAD_SAFE_FUNCTIONS >= 20112L ) )
+/*
+ * This is a convenience shorthand macro to avoid checking the long
+ * preprocessor conditions above. Ideally, we could expose this macro in
+ * platform_util.h and simply use it in platform_util.c, threading.c and
+ * threading.h. However, this macro is not part of the Mbed TLS public API, so
+ * we keep it private by only defining it in this file
+ */
+#if ! ( defined(_WIN32) && !defined(EFIX64) && !defined(EFI32) )
+#define PLATFORM_UTIL_USE_GMTIME
+#endif /* ! ( defined(_WIN32) && !defined(EFIX64) && !defined(EFI32) ) */
+
+#endif /* !( ( defined(_POSIX_VERSION) && _POSIX_VERSION >= 200809L ) ||     \
+             ( defined(_POSIX_THREAD_SAFE_FUNCTIONS ) &&                     \
+                _POSIX_THREAD_SAFE_FUNCTIONS >= 20112L ) ) */
+
+struct tm *mbedtls_platform_gmtime_r( const mbedtls_time_t *tt,
+                                      struct tm *tm_buf )
+{
+#if defined(_WIN32) && !defined(EFIX64) && !defined(EFI32)
+    return( ( gmtime_s( tm_buf, tt ) == 0 ) ? tm_buf : NULL );
+#elif !defined(PLATFORM_UTIL_USE_GMTIME)
+    return( gmtime_r( tt, tm_buf ) );
+#else
+    struct tm *lt;
+
+#if defined(MBEDTLS_THREADING_C)
+    if( mbedtls_mutex_lock( &mbedtls_threading_gmtime_mutex ) != 0 )
+        return( NULL );
+#endif /* MBEDTLS_THREADING_C */
+
+    lt = gmtime( tt );
+
+    if( lt != NULL )
+    {
+        memcpy( tm_buf, lt, sizeof( struct tm ) );
+    }
+
+#if defined(MBEDTLS_THREADING_C)
+    if( mbedtls_mutex_unlock( &mbedtls_threading_gmtime_mutex ) != 0 )
+        return( NULL );
+#endif /* MBEDTLS_THREADING_C */
+
+    return( ( lt == NULL ) ? NULL : tm_buf );
+#endif /* _WIN32 && !EFIX64 && !EFI32 */
+}
+#endif /* MBEDTLS_HAVE_TIME_DATE && MBEDTLS_PLATFORM_GMTIME_R_ALT */
diff --git a/library/threading.c b/library/threading.c
index 7a32e67..7c90c7c 100644
--- a/library/threading.c
+++ b/library/threading.c
@@ -19,6 +19,14 @@
  *  This file is part of mbed TLS (https://tls.mbed.org)
  */
 
+/*
+ * Ensure gmtime_r is available even with -std=c99; must be defined before
+ * config.h, which pulls in glibc's features.h. Harmless on other platforms.
+ */
+#if !defined(_POSIX_C_SOURCE)
+#define _POSIX_C_SOURCE 200112L
+#endif
+
 #if !defined(MBEDTLS_CONFIG_FILE)
 #include "mbedtls/config.h"
 #else
@@ -29,6 +37,36 @@
 
 #include "mbedtls/threading.h"
 
+#if defined(MBEDTLS_HAVE_TIME_DATE) && !defined(MBEDTLS_PLATFORM_GMTIME_R_ALT)
+
+#if !defined(_WIN32) && (defined(unix) || \
+    defined(__unix) || defined(__unix__) || (defined(__APPLE__) && \
+    defined(__MACH__)))
+#include <unistd.h>
+#endif /* !_WIN32 && (unix || __unix || __unix__ ||
+        * (__APPLE__ && __MACH__)) */
+
+#if !( ( defined(_POSIX_VERSION) && _POSIX_VERSION >= 200809L ) ||     \
+       ( defined(_POSIX_THREAD_SAFE_FUNCTIONS ) &&                     \
+         _POSIX_THREAD_SAFE_FUNCTIONS >= 20112L ) )
+/*
+ * This is a convenience shorthand macro to avoid checking the long
+ * preprocessor conditions above. Ideally, we could expose this macro in
+ * platform_util.h and simply use it in platform_util.c, threading.c and
+ * threading.h. However, this macro is not part of the Mbed TLS public API, so
+ * we keep it private by only defining it in this file
+ */
+
+#if ! ( defined(_WIN32) && !defined(EFIX64) && !defined(EFI32) )
+#define THREADING_USE_GMTIME
+#endif /* ! ( defined(_WIN32) && !defined(EFIX64) && !defined(EFI32) ) */
+
+#endif /* !( ( defined(_POSIX_VERSION) && _POSIX_VERSION >= 200809L ) ||     \
+             ( defined(_POSIX_THREAD_SAFE_FUNCTIONS ) &&                     \
+                _POSIX_THREAD_SAFE_FUNCTIONS >= 20112L ) ) */
+
+#endif /* MBEDTLS_HAVE_TIME_DATE && !MBEDTLS_PLATFORM_GMTIME_R_ALT */
+
 #if defined(MBEDTLS_THREADING_PTHREAD)
 static void threading_mutex_init_pthread( mbedtls_threading_mutex_t *mutex )
 {
@@ -114,6 +152,9 @@
 #if defined(MBEDTLS_FS_IO)
     mbedtls_mutex_init( &mbedtls_threading_readdir_mutex );
 #endif
+#if defined(THREADING_USE_GMTIME)
+    mbedtls_mutex_init( &mbedtls_threading_gmtime_mutex );
+#endif
 }
 
 /*
@@ -124,6 +165,9 @@
 #if defined(MBEDTLS_FS_IO)
     mbedtls_mutex_free( &mbedtls_threading_readdir_mutex );
 #endif
+#if defined(THREADING_USE_GMTIME)
+    mbedtls_mutex_free( &mbedtls_threading_gmtime_mutex );
+#endif
 }
 #endif /* MBEDTLS_THREADING_ALT */
 
@@ -136,5 +180,8 @@
 #if defined(MBEDTLS_FS_IO)
 mbedtls_threading_mutex_t mbedtls_threading_readdir_mutex MUTEX_INIT;
 #endif
+#if defined(THREADING_USE_GMTIME)
+mbedtls_threading_mutex_t mbedtls_threading_gmtime_mutex MUTEX_INIT;
+#endif
 
 #endif /* MBEDTLS_THREADING_C */
diff --git a/library/x509.c b/library/x509.c
index 2e6795f..52b5b64 100644
--- a/library/x509.c
+++ b/library/x509.c
@@ -29,10 +29,6 @@
  *  http://www.itu.int/ITU-T/studygroups/com17/languages/X.690-0207.pdf
  */
 
-/* Ensure gmtime_r is available even with -std=c99; must be included before
- * config.h, which pulls in glibc's features.h. Harmless on other platforms. */
-#define _POSIX_C_SOURCE 200112L
-
 #if !defined(MBEDTLS_CONFIG_FILE)
 #include "mbedtls/config.h"
 #else
@@ -67,6 +63,7 @@
 #include "mbedtls/platform_time.h"
 #endif
 #if defined(MBEDTLS_HAVE_TIME_DATE)
+#include "mbedtls/platform_util.h"
 #include <time.h>
 #endif
 
@@ -901,11 +898,7 @@
     int ret = 0;
 
     tt = mbedtls_time( NULL );
-#if defined(_WIN32) && !defined(EFIX64) && !defined(EFI32)
-    lt = gmtime_s( &tm_buf, &tt ) == 0 ? &tm_buf : NULL;
-#else
-    lt = gmtime_r( &tt, &tm_buf );
-#endif
+    lt = mbedtls_platform_gmtime_r( &tt, &tm_buf );
 
     if( lt == NULL )
         ret = -1;