diff --git a/ChangeLog b/ChangeLog
index f5721b0..49b6ec7 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -56,10 +56,14 @@
    * Correct the documentation for `mbedtls_ssl_get_session()`.
      This API has deep copy of the session, and the peer
      certificate is not lost. Fixes #926.
+   * Fix build using -std=c99. Fixed by Nick Wilson.
 
 Changes
    * Change the shebang line in Perl scripts to look up perl in the PATH.
      Contributed by fbrosson.
+   * Allow overriding the time on Windows via the platform-time abstraction.
+     Fixed by Nick Wilson.
+   * Use gmtime_r/gmtime_s for thread-safety. Fixed by Nick Wilson.
 
 = mbed TLS 2.11.0 branch released 2018-06-18
 
diff --git a/include/mbedtls/threading.h b/include/mbedtls/threading.h
index aeea5d0..c25daa5 100644
--- a/include/mbedtls/threading.h
+++ b/include/mbedtls/threading.h
@@ -99,9 +99,6 @@
 #if defined(MBEDTLS_FS_IO)
 extern mbedtls_threading_mutex_t mbedtls_threading_readdir_mutex;
 #endif
-#if defined(MBEDTLS_HAVE_TIME_DATE)
-extern mbedtls_threading_mutex_t mbedtls_threading_gmtime_mutex;
-#endif
 #endif /* MBEDTLS_THREADING_C */
 
 #ifdef __cplusplus
diff --git a/library/entropy_poll.c b/library/entropy_poll.c
index 308c53c..040aa11 100644
--- a/library/entropy_poll.c
+++ b/library/entropy_poll.c
@@ -19,6 +19,11 @@
  *  This file is part of mbed TLS (https://tls.mbed.org)
  */
 
+#if defined(__linux__)
+/* Ensure that syscall() is available even when compiling with -std=c99 */
+#define _GNU_SOURCE
+#endif
+
 #if !defined(MBEDTLS_CONFIG_FILE)
 #include "mbedtls/config.h"
 #else
diff --git a/library/net_sockets.c b/library/net_sockets.c
index e5efce0..816b130 100644
--- a/library/net_sockets.c
+++ b/library/net_sockets.c
@@ -19,6 +19,11 @@
  *  This file is part of mbed TLS (https://tls.mbed.org)
  */
 
+/* Enable definition of getaddrinfo() even when compiling with -std=c99. Must
+ * be set before config.h, which pulls in glibc's features.h indirectly.
+ * Harmless on other platforms. */
+#define _POSIX_C_SOURCE 200112L
+
 #if !defined(MBEDTLS_CONFIG_FILE)
 #include "mbedtls/config.h"
 #else
diff --git a/library/threading.c b/library/threading.c
index f1c3724..7a32e67 100644
--- a/library/threading.c
+++ b/library/threading.c
@@ -114,9 +114,6 @@
 #if defined(MBEDTLS_FS_IO)
     mbedtls_mutex_init( &mbedtls_threading_readdir_mutex );
 #endif
-#if defined(MBEDTLS_HAVE_TIME_DATE)
-    mbedtls_mutex_init( &mbedtls_threading_gmtime_mutex );
-#endif
 }
 
 /*
@@ -127,9 +124,6 @@
 #if defined(MBEDTLS_FS_IO)
     mbedtls_mutex_free( &mbedtls_threading_readdir_mutex );
 #endif
-#if defined(MBEDTLS_HAVE_TIME_DATE)
-    mbedtls_mutex_free( &mbedtls_threading_gmtime_mutex );
-#endif
 }
 #endif /* MBEDTLS_THREADING_ALT */
 
@@ -142,8 +136,5 @@
 #if defined(MBEDTLS_FS_IO)
 mbedtls_threading_mutex_t mbedtls_threading_readdir_mutex MUTEX_INIT;
 #endif
-#if defined(MBEDTLS_HAVE_TIME_DATE)
-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 264c7fb..2e6795f 100644
--- a/library/x509.c
+++ b/library/x509.c
@@ -29,6 +29,10 @@
  *  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
@@ -59,14 +63,10 @@
 #define mbedtls_snprintf  snprintf
 #endif
 
-
 #if defined(MBEDTLS_HAVE_TIME)
 #include "mbedtls/platform_time.h"
 #endif
-
-#if defined(_WIN32) && !defined(EFIX64) && !defined(EFI32)
-#include <windows.h>
-#else
+#if defined(MBEDTLS_HAVE_TIME_DATE)
 #include <time.h>
 #endif
 
@@ -894,36 +894,18 @@
  * Set the time structure to the current time.
  * Return 0 on success, non-zero on failure.
  */
-#if defined(_WIN32) && !defined(EFIX64) && !defined(EFI32)
 static int x509_get_current_time( mbedtls_x509_time *now )
 {
-    SYSTEMTIME st;
-
-    GetSystemTime( &st );
-
-    now->year = st.wYear;
-    now->mon  = st.wMonth;
-    now->day  = st.wDay;
-    now->hour = st.wHour;
-    now->min  = st.wMinute;
-    now->sec  = st.wSecond;
-
-    return( 0 );
-}
-#else
-static int x509_get_current_time( mbedtls_x509_time *now )
-{
-    struct tm *lt;
+    struct tm *lt, tm_buf;
     mbedtls_time_t tt;
     int ret = 0;
 
-#if defined(MBEDTLS_THREADING_C)
-    if( mbedtls_mutex_lock( &mbedtls_threading_gmtime_mutex ) != 0 )
-        return( MBEDTLS_ERR_THREADING_MUTEX_ERROR );
-#endif
-
     tt = mbedtls_time( NULL );
-    lt = gmtime( &tt );
+#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
 
     if( lt == NULL )
         ret = -1;
@@ -937,14 +919,8 @@
         now->sec  = lt->tm_sec;
     }
 
-#if defined(MBEDTLS_THREADING_C)
-    if( mbedtls_mutex_unlock( &mbedtls_threading_gmtime_mutex ) != 0 )
-        return( MBEDTLS_ERR_THREADING_MUTEX_ERROR );
-#endif
-
     return( ret );
 }
-#endif /* _WIN32 && !EFIX64 && !EFI32 */
 
 /*
  * Return 0 if before <= after, 1 otherwise
diff --git a/programs/aes/aescrypt2.c b/programs/aes/aescrypt2.c
index c727f93..69c4060 100644
--- a/programs/aes/aescrypt2.c
+++ b/programs/aes/aescrypt2.c
@@ -19,6 +19,11 @@
  *  This file is part of mbed TLS (https://tls.mbed.org)
  */
 
+/* Enable definition of fileno() even when compiling with -std=c99. Must be
+ * set before config.h, which pulls in glibc's features.h indirectly.
+ * Harmless on other platforms. */
+#define _POSIX_C_SOURCE 1
+
 #if !defined(MBEDTLS_CONFIG_FILE)
 #include "mbedtls/config.h"
 #else
diff --git a/programs/aes/crypt_and_hash.c b/programs/aes/crypt_and_hash.c
index 99d30c9..bc95eb9 100644
--- a/programs/aes/crypt_and_hash.c
+++ b/programs/aes/crypt_and_hash.c
@@ -20,6 +20,11 @@
  *  This file is part of mbed TLS (https://tls.mbed.org)
  */
 
+/* Enable definition of fileno() even when compiling with -std=c99. Must be
+ * set before config.h, which pulls in glibc's features.h indirectly.
+ * Harmless on other platforms. */
+#define _POSIX_C_SOURCE 1
+
 #if !defined(MBEDTLS_CONFIG_FILE)
 #include "mbedtls/config.h"
 #else
diff --git a/programs/ssl/ssl_mail_client.c b/programs/ssl/ssl_mail_client.c
index 04f8910..d3b569c 100644
--- a/programs/ssl/ssl_mail_client.c
+++ b/programs/ssl/ssl_mail_client.c
@@ -19,6 +19,11 @@
  *  This file is part of mbed TLS (https://tls.mbed.org)
  */
 
+/* Enable definition of gethostname() even when compiling with -std=c99. Must
+ * be set before config.h, which pulls in glibc's features.h indirectly.
+ * Harmless on other platforms. */
+#define _POSIX_C_SOURCE 200112L
+
 #if !defined(MBEDTLS_CONFIG_FILE)
 #include "mbedtls/config.h"
 #else
diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index 28331ba..52632f8 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -15,6 +15,11 @@
     message(FATAL_ERROR "Cannot build test suites without Perl")
 endif()
 
+# Enable definition of various functions used throughout the testsuite
+# (gethostname, strdup, fileno...) even when compiling with -std=c99. Harmless
+# on non-POSIX platforms.
+add_definitions("-D_POSIX_C_SOURCE=200809L")
+
 function(add_test_suite suite_name)
     if(ARGV1)
         set(data_name ${ARGV1})
diff --git a/tests/Makefile b/tests/Makefile
index a592d9e..3632554 100644
--- a/tests/Makefile
+++ b/tests/Makefile
@@ -12,6 +12,11 @@
 		-lmbedx509$(SHARED_SUFFIX)	\
 		-lmbedcrypto$(SHARED_SUFFIX)
 
+# Enable definition of various functions used throughout the testsuite
+# (gethostname, strdup, fileno...) even when compiling with -std=c99. Harmless
+# on non-POSIX platforms.
+LOCAL_CFLAGS += -D_POSIX_C_SOURCE=200809L
+
 ifndef SHARED
 DEP=../library/libmbedcrypto.a ../library/libmbedx509.a ../library/libmbedtls.a
 else
diff --git a/tests/suites/helpers.function b/tests/suites/helpers.function
index f82694a..8f04885 100644
--- a/tests/suites/helpers.function
+++ b/tests/suites/helpers.function
@@ -36,6 +36,7 @@
 
 #if defined(__unix__) || (defined(__APPLE__) && defined(__MACH__))
 #include <unistd.h>
+#include <strings.h>
 #endif
 
 /*----------------------------------------------------------------------------*/