https://www.illumos.org/issues/17380

--- spdlog-1.17.0/include/spdlog/details/os-inl.h.orig
+++ spdlog-1.17.0/include/spdlog/details/os-inl.h
@@ -244,7 +244,7 @@
 #endif
 
 #if !defined(SPDLOG_NO_TZ_OFFSET)
-#ifdef _WIN32
+#ifdef __illumos__
 // Compare the timestamp as Local (mktime) vs UTC (_mkgmtime) to get the offset.
 SPDLOG_INLINE int utc_minutes_offset(const std::tm &tm) {
     std::tm local_tm = tm;  // copy since mktime might adjust it (normalize dates, set tm_isdst)
@@ -253,7 +253,7 @@
         return 0; // fallback
     }
 
-    std::time_t utc_time_t = _mkgmtime(&local_tm);
+    std::time_t utc_time_t = timegm(&local_tm);
     if (utc_time_t == -1) {
         return 0; // fallback
     }