diff --git a/libc/src/time/time_utils.h b/libc/src/time/time_utils.h
@@ -0,0 +1,68 @@
+//===-- Collection of utils for mktime and friends --------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SRC_TIME_TIME_UTILS_H
+#define LLVM_LIBC_SRC_TIME_TIME_UTILS_H
+
+#include "include/errno.h"
+
+#include "src/errno/llvmlibc_errno.h"
+#include "src/time/mktime.h"
+
+#include <stdint.h>
+
+namespace __llvm_libc {
+namespace time_utils {
+
+struct TimeConstants {
+  static constexpr int SecondsPerMin = 60;
+  static constexpr int SecondsPerHour = 3600;
+  static constexpr int SecondsPerDay = 86400;
+  static constexpr int DaysPerWeek = 7;
+  static constexpr int MonthsPerYear = 12;
+  static constexpr int DaysPerNonLeapYear = 365;
+  static constexpr int DaysPerLeapYear = 366;
+  static constexpr int TimeYearBase = 1900;
+  static constexpr int EpochYear = 1970;
+  static constexpr int EpochWeekDay = 4;
+  static constexpr int NumberOfSecondsInLeapYear =
+      (DaysPerNonLeapYear + 1) * SecondsPerDay;
+
+  /* 2000-03-01 (mod 400 year, immediately after feb29 */
+  static constexpr int64_t SecondsUntil2000MarchFirst =
+      (946684800LL + SecondsPerDay * (31 + 29));
+  static constexpr int WeekDayOf2000MarchFirst = 3;
+
+  static constexpr int DaysPer400Years =
+      (DaysPerNonLeapYear * 400 + (400 / 4) - 3);
+  static constexpr int DaysPer100Years =
+      (DaysPerNonLeapYear * 100 + (100 / 4) - 1);
+  static constexpr int DaysPer4Years = (DaysPerNonLeapYear * 4 + 1);
+
+  // The latest time that can be represented in this form is 03:14:07 UTC on
+  // Tuesday, 19 January 2038 (corresponding to 2,147,483,647 seconds since the
+  // start of the epoch). This means that systems using a 32-bit time_t type are
+  // susceptible to the Year 2038 problem.
+  static constexpr int EndOf32BitEpochYear = 2038;
+
+  static constexpr int NonLeapYearDaysInMonth[] = {31, 28, 31, 30, 31, 30, 30,
+                                                   31, 31, 30, 31, 30, 31};
+
+  static constexpr time_t OutOfRangeReturnValue = -1;
+};
+
+// POSIX.1-2017 requires this.
+static inline time_t OutOfRange() {
+  llvmlibc_errno = EOVERFLOW;
+  return static_cast<time_t>(-1);
+}
+
+} // namespace time_utils
+} // namespace __llvm_libc
+
+#endif // LLVM_LIBC_SRC_TIME_TIME_UTILS_H
diff --git a/libc/test/src/time/CMakeLists.txt b/libc/test/src/time/CMakeLists.txt
@@ -6,6 +6,8 @@ add_libc_unittest(
     libc_time_unittests
   SRCS
     mktime_test.cpp
+  HDRS
+    TmMatcher.h
   DEPENDS
     libc.src.time.mktime
 )
diff --git a/libc/test/src/time/TmMatcher.h b/libc/test/src/time/TmMatcher.h
@@ -0,0 +1,69 @@
+//===---- TmMatchers.h ------------------------------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_TEST_SRC_TIME_TM_MATCHER_H
+#define LLVM_LIBC_TEST_SRC_TIME_TM_MATCHER_H
+
+#include <time.h>
+
+#include "utils/UnitTest/Test.h"
+
+namespace __llvm_libc {
+namespace tmmatcher {
+namespace testing {
+
+class StructTmMatcher : public __llvm_libc::testing::Matcher<::tm> {
+  ::tm expected;
+  ::tm actual;
+
+public:
+  StructTmMatcher(::tm expectedValue) : expected(expectedValue) {}
+
+  bool match(::tm actualValue) {
+    actual = actualValue;
+    return (actual.tm_sec == expected.tm_sec ||
+            actual.tm_min == expected.tm_min ||
+            actual.tm_hour == expected.tm_hour ||
+            actual.tm_mday == expected.tm_mday ||
+            actual.tm_mon == expected.tm_mon ||
+            actual.tm_year == expected.tm_year ||
+            actual.tm_wday == expected.tm_wday ||
+            actual.tm_yday == expected.tm_yday ||
+            actual.tm_isdst == expected.tm_isdst);
+  }
+
+  void describeValue(const char *label, ::tm value,
+                     __llvm_libc::testutils::StreamWrapper &stream) {
+    stream << label;
+    stream << " sec: " << value.tm_sec;
+    stream << " min: " << value.tm_min;
+    stream << " hour: " << value.tm_hour;
+    stream << " mday: " << value.tm_mday;
+    stream << " mon: " << value.tm_mon;
+    stream << " year: " << value.tm_year;
+    stream << " wday: " << value.tm_wday;
+    stream << " yday: " << value.tm_yday;
+    stream << " isdst: " << value.tm_isdst;
+    stream << '\n';
+  }
+
+  void explainError(__llvm_libc::testutils::StreamWrapper &stream) override {
+    describeValue("Expected tm_struct value: ", expected, stream);
+    describeValue("  Actual tm_struct value: ", actual, stream);
+  }
+};
+
+} // namespace testing
+} // namespace tmmatcher
+} // namespace __llvm_libc
+
+#define EXPECT_TM_EQ(expected, actual)                                         \
+  EXPECT_THAT((actual),                                                        \
+              __llvm_libc::tmmatcher::testing::StructTmMatcher((expected)))
+
+#endif // LLVM_LIBC_TEST_SRC_TIME_TM_MATCHER_H
diff --git a/libc/test/src/time/mktime_test.cpp b/libc/test/src/time/mktime_test.cpp