diff --git a/flang-rt/lib/runtime/extensions.cpp b/flang-rt/lib/runtime/extensions.cpp
index f6c39468d5655..bc86917c00828 100644
--- a/flang-rt/lib/runtime/extensions.cpp
+++ b/flang-rt/lib/runtime/extensions.cpp
@@ -18,6 +18,7 @@
 #include "flang/Runtime/entry-names.h"
 #include "flang/Runtime/io-api.h"
 #include "flang/Runtime/iostat-consts.h"
+#include <atomic>
 #include <chrono>
 #include <cstdio>
 #include <cstring>
@@ -303,6 +304,75 @@ void FORTRAN_PROCEDURE_NAME(qsort)(int *array, int *len, int *isize,
 // PERROR(STRING)
 void RTNAME(Perror)(const char *str) { perror(str); }
 
+// GNU extension function SECNDS(refTime)
+float FORTRAN_PROCEDURE_NAME(secnds)(float *refTime) {
+  constexpr float FAIL_SECNDS{-1.0f}; // Failure code for this function
+  // Failure code for time functions that return std::time_t
+  constexpr std::time_t FAIL_TIME{std::time_t{-1}};
+  constexpr std::time_t TIME_UNINITIALIZED{std::time_t{0}};
+  if (!refTime) {
+    return FAIL_SECNDS;
+  }
+  std::time_t now{std::time(nullptr)};
+  if (now == FAIL_TIME) {
+    return FAIL_SECNDS;
+  }
+  // In float result, we can only precisely store 2^24 seconds, which
+  // comes out to about 194 days. Thus, need to pick a starting point,
+  // which will allow us to keep the time diffs as precise as possible.
+  // Given the description of this function, midnight of the current
+  // day is the best starting point.
+  static std::atomic<std::time_t> startingPoint{TIME_UNINITIALIZED};
+  // "Acquire" will give us writes from other threads.
+  std::time_t localStartingPoint{startingPoint.load(std::memory_order_acquire)};
+  // Initialize startingPoint if we haven't initialized it yet or
+  // if we were passed 0.0f, which indicates to compute seconds from
+  // current day's midnight.
+  if (localStartingPoint == TIME_UNINITIALIZED || *refTime < 0.5f) {
+    // Compute midnight in the current timezone and try to initialize
+    // startingPoint with it. If there are any errors during computation,
+    // exit with error and hope that the other threads have better luck
+    // (or the user retries the call).
+    struct tm timeInfo;
+#ifdef _WIN32
+    if (localtime_s(&timeInfo, &now)) {
+#else
+    if (!localtime_r(&now, &timeInfo)) {
+#endif
+      return FAIL_SECNDS;
+    }
+    // Back to midnight
+    timeInfo.tm_hour = 0;
+    timeInfo.tm_min = 0;
+    timeInfo.tm_sec = 0;
+    localStartingPoint = std::mktime(&timeInfo);
+    if (localStartingPoint == FAIL_TIME) {
+      return FAIL_SECNDS;
+    }
+    INTERNAL_CHECK(localStartingPoint > TIME_UNINITIALIZED);
+    // Attempt to atomically set startingPoint to localStartingPoint
+    std::time_t expected{TIME_UNINITIALIZED};
+    if (startingPoint.compare_exchange_strong(expected, localStartingPoint,
+            std::memory_order_acq_rel, // "Acquire and release" on success
+            std::memory_order_acquire)) { // "Acquire" on failure
+      // startingPoint was set to localStartingPoint
+    } else {
+      // startingPoint was already initialized and its value was loaded
+      // into `expected`. Discard our precomputed midnight value in favor
+      // of the one from startingPoint.
+      localStartingPoint = expected;
+    }
+  }
+  double diffStartingPoint{std::difftime(now, localStartingPoint)};
+  return static_cast<float>(diffStartingPoint) - *refTime;
+}
+
+float RTNAME(Secnds)(float *refTime, const char *sourceFile, int line) {
+  Terminator terminator{sourceFile, line};
+  RUNTIME_CHECK(terminator, refTime != nullptr);
+  return FORTRAN_PROCEDURE_NAME(secnds)(refTime);
+}
+
 // GNU extension function TIME()
 std::int64_t RTNAME(time)() { return time(nullptr); }
 
diff --git a/flang/include/flang/Runtime/extensions.h b/flang/include/flang/Runtime/extensions.h
index b350204714431..9a100cec9e6b9 100644
--- a/flang/include/flang/Runtime/extensions.h
+++ b/flang/include/flang/Runtime/extensions.h
@@ -90,5 +90,9 @@ void RTNAME(Perror)(const char *str);
 // MCLOCK -- returns accumulated time in ticks
 int FORTRAN_PROCEDURE_NAME(mclock)();
 
+// GNU extension subroutine SECNDS(refTime)
+float FORTRAN_PROCEDURE_NAME(secnds)(float *refTime);
+float RTNAME(Secnds)(float *refTime, const char *sourceFile, int line);
+
 } // extern "C"
 #endif // FORTRAN_RUNTIME_EXTENSIONS_H_