9 files changed, 64 insertions, 5 deletions
diff --git a/src/build-data/os/cygwin.txt b/src/build-data/os/cygwin.txt
index eb5835a6f..be67e2607 100644
--- a/src/build-data/os/cygwin.txt
+++ b/src/build-data/os/cygwin.txt
@@ -13,4 +13,5 @@ doc_dir docs
 <target_features>
 gettimeofday
 getsid
+timegm
 </target_features>
diff --git a/src/build-data/os/darwin.txt b/src/build-data/os/darwin.txt
index f5f2589fb..3a13b34e4 100644
--- a/src/build-data/os/darwin.txt
+++ b/src/build-data/os/darwin.txt
@@ -15,6 +15,7 @@ gmtime_r
 memset_s
 readdir
 getsid
+timegm
 </target_features>
 
 <aliases>
diff --git a/src/build-data/os/freebsd.txt b/src/build-data/os/freebsd.txt
index 346145785..32767cb1f 100644
--- a/src/build-data/os/freebsd.txt
+++ b/src/build-data/os/freebsd.txt
@@ -8,4 +8,5 @@ gmtime_r
 dlopen
 readdir
 getsid
+timegm
 </target_features>
diff --git a/src/build-data/os/linux.txt b/src/build-data/os/linux.txt
index cb78c33ff..48c3bf318 100644
--- a/src/build-data/os/linux.txt
+++ b/src/build-data/os/linux.txt
@@ -8,6 +8,7 @@ gmtime_r
 dlopen
 readdir
 getsid
+timegm
 </target_features>
 
 <aliases>
diff --git a/src/build-data/os/mingw.txt b/src/build-data/os/mingw.txt
index 4f3e7f08b..cc98b11e6 100644
--- a/src/build-data/os/mingw.txt
+++ b/src/build-data/os/mingw.txt
@@ -23,6 +23,7 @@ mingw32
 <target_features>
 cryptgenrandom
 loadlibrary
+mkgmtime
 win32_virtual_lock
 win32_get_systemtime
 </target_features>
diff --git a/src/build-data/os/netbsd.txt b/src/build-data/os/netbsd.txt
index 346145785..32767cb1f 100644
--- a/src/build-data/os/netbsd.txt
+++ b/src/build-data/os/netbsd.txt
@@ -8,4 +8,5 @@ gmtime_r
 dlopen
 readdir
 getsid
+timegm
 </target_features>
diff --git a/src/build-data/os/openbsd.txt b/src/build-data/os/openbsd.txt
index 5df133c8e..510a7accd 100644
--- a/src/build-data/os/openbsd.txt
+++ b/src/build-data/os/openbsd.txt
@@ -7,4 +7,5 @@ posix_mlock
 gmtime_r
 dlopen
 readdir
+timegm
 </target_features>
diff --git a/src/build-data/os/windows.txt b/src/build-data/os/windows.txt
index c34f2e7ab..fd74fe817 100644
--- a/src/build-data/os/windows.txt
+++ b/src/build-data/os/windows.txt
@@ -15,6 +15,7 @@ install_cmd_exec "copy"
 cryptgenrandom
 gmtime_s
 loadlibrary
+mkgmtime
 query_perf_counter
 virtual_lock
 rtlsecurezeromemory
diff --git a/src/lib/utils/calendar.cpp b/src/lib/utils/calendar.cpp
index f62303e49..966c97730 100644
--- a/src/lib/utils/calendar.cpp
+++ b/src/lib/utils/calendar.cpp
@@ -11,6 +11,7 @@
 #include <ctime>
 #include <sstream>
 #include <iomanip>
+#include <mutex>
 
 namespace Botan {
 
@@ -34,11 +35,53 @@ std::tm do_gmtime(std::time_t time_val)
    return tm;
    }
 
+#if !defined(BOTAN_TARGET_OS_HAS_TIMEGM) && !defined(BOTAN_TARGET_OS_HAS_MKGMTIME)
+
+#pragma message "Caution! A fallback version of timegm() is used which is not thread-safe"
+
+std::mutex ENV_TZ;
+
+std::time_t fallback_timegm(std::tm *tm)
+   {
+   std::time_t out;
+   std::string tz_backup;
+
+   ENV_TZ.lock();
+
+   // Store current value of env variable TZ
+   const char* tz_env_pointer = ::getenv("TZ");
+   if (tz_env_pointer != nullptr)
+      tz_backup = std::string(tz_env_pointer);
+
+   // Clear value of TZ
+   ::setenv("TZ", "", 1);
+   ::tzset();
+   
+   out = ::mktime(tm);
+
+   // Restore TZ
+   if (!tz_backup.empty())
+      {
+      // setenv makes a copy of the second argument
+      ::setenv("TZ", tz_backup.data(), 1);
+      }
+   else
+      {
+      ::unsetenv("TZ");
+      }
+   ::tzset();
+
+   ENV_TZ.unlock();
+
+   return out;
+}
+#endif
+
 }
 
 std::chrono::system_clock::time_point calendar_point::to_std_timepoint()
    {
-   if (year < 1900)
+   if (year < 1970)
       throw Invalid_Argument("calendar_point::to_std_timepoint() does not support years before 1990.");
 
    // 32 bit time_t ends at January 19, 2038
@@ -48,7 +91,9 @@ std::chrono::system_clock::time_point calendar_point::to_std_timepoint()
    if (year > 2037)
       throw Invalid_Argument("calendar_point::to_std_timepoint() does not support years after 2037.");
 
+   // std::tm: struct without any timezone information
    std::tm tm;
+   tm.tm_isdst = -1; // i.e. no DST information available
    tm.tm_sec   = seconds;
    tm.tm_min   = minutes;
    tm.tm_hour  = hour;
@@ -56,12 +101,18 @@ std::chrono::system_clock::time_point calendar_point::to_std_timepoint()
    tm.tm_mon   = month - 1;
    tm.tm_year  = year - 1900;
 
-   // Convert std::tm to std::time_t
+   // Define a function alias `botan_timegm`
+   #if defined(BOTAN_TARGET_OS_HAS_TIMEGM)
+   std::time_t (&botan_timegm)(std::tm *tm) = timegm;
+   #elif defined(BOTAN_TARGET_OS_HAS_MKGMTIME)
    // http://stackoverflow.com/questions/16647819/timegm-cross-platform
-   #if defined(BOTAN_TARGET_OS_IS_WINDOWS)
-   #define timegm _mkgmtime
+   std::time_t (&botan_timegm)(std::tm *tm) = _mkgmtime;
+   #else
+   std::time_t (&botan_timegm)(std::tm *tm) = fallback_timegm;
    #endif
-   std::time_t tt = timegm(&tm);
+
+   // Convert std::tm to std::time_t
+   std::time_t tt = botan_timegm(&tm);
    if (tt == -1)
       throw Invalid_Argument("calendar_point couldn't be converted: " + to_string());