diff --git a/libutils/Android.bp b/libutils/Android.bp
index 0f7044a..ea39d34 100644
--- a/libutils/Android.bp
+++ b/libutils/Android.bp
@@ -246,6 +246,7 @@
         "String8_test.cpp",
         "String16_test.cpp",
         "StrongPointer_test.cpp",
+        "Timers_test.cpp",
         "Unicode_test.cpp",
         "Vector_test.cpp",
     ],
diff --git a/libutils/Timers.cpp b/libutils/Timers.cpp
index 1172ae7..fd3f4a9 100644
--- a/libutils/Timers.cpp
+++ b/libutils/Timers.cpp
@@ -20,31 +20,37 @@
 #include <utils/Timers.h>
 
 #include <limits.h>
+#include <stdlib.h>
 #include <time.h>
 
-// host linux support requires Linux 2.6.39+
+#include <android-base/macros.h>
+
+static constexpr size_t clock_id_max = 5;
+
+static void checkClockId(int clock) {
+    if (clock < 0 || clock >= clock_id_max) abort();
+}
+
 #if defined(__linux__)
-nsecs_t systemTime(int clock)
-{
-    static const clockid_t clocks[] = {
-            CLOCK_REALTIME,
-            CLOCK_MONOTONIC,
-            CLOCK_PROCESS_CPUTIME_ID,
-            CLOCK_THREAD_CPUTIME_ID,
-            CLOCK_BOOTTIME
-    };
-    struct timespec t;
-    t.tv_sec = t.tv_nsec = 0;
+nsecs_t systemTime(int clock) {
+    checkClockId(clock);
+    static constexpr clockid_t clocks[] = {CLOCK_REALTIME, CLOCK_MONOTONIC,
+                                           CLOCK_PROCESS_CPUTIME_ID, CLOCK_THREAD_CPUTIME_ID,
+                                           CLOCK_BOOTTIME};
+    static_assert(clock_id_max == arraysize(clocks));
+    timespec t = {};
     clock_gettime(clocks[clock], &t);
     return nsecs_t(t.tv_sec)*1000000000LL + t.tv_nsec;
 }
 #else
-nsecs_t systemTime(int /*clock*/)
-{
+nsecs_t systemTime(int clock) {
+    // TODO: is this ever called with anything but REALTIME on mac/windows?
+    checkClockId(clock);
+
     // Clock support varies widely across hosts. Mac OS doesn't support
-    // CLOCK_BOOTTIME, and Windows is windows.
-    struct timeval t;
-    t.tv_sec = t.tv_usec = 0;
+    // CLOCK_BOOTTIME (and doesn't even have clock_gettime until 10.12).
+    // Windows is windows.
+    timeval t = {};
     gettimeofday(&t, nullptr);
     return nsecs_t(t.tv_sec)*1000000000LL + nsecs_t(t.tv_usec)*1000LL;
 }
diff --git a/libutils/Timers_test.cpp b/libutils/Timers_test.cpp
new file mode 100644
index 0000000..ec0051e
--- /dev/null
+++ b/libutils/Timers_test.cpp
@@ -0,0 +1,29 @@
+/*
+ * Copyright (C) 2020 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include <utils/Timers.h>
+
+#include <gtest/gtest.h>
+
+TEST(Timers, systemTime_invalid) {
+    EXPECT_EXIT(systemTime(-1), testing::KilledBySignal(SIGABRT), "");
+    systemTime(SYSTEM_TIME_REALTIME);
+    systemTime(SYSTEM_TIME_MONOTONIC);
+    systemTime(SYSTEM_TIME_PROCESS);
+    systemTime(SYSTEM_TIME_THREAD);
+    systemTime(SYSTEM_TIME_BOOTTIME);
+    EXPECT_EXIT(systemTime(SYSTEM_TIME_BOOTTIME + 1), testing::KilledBySignal(SIGABRT), "");
+}
diff --git a/libutils/include/utils/Timers.h b/libutils/include/utils/Timers.h
index 54ec474..197fc26 100644
--- a/libutils/include/utils/Timers.h
+++ b/libutils/include/utils/Timers.h
@@ -14,11 +14,7 @@
  * limitations under the License.
  */
 
-//
-// Timer functions.
-//
-#ifndef _LIBS_UTILS_TIMERS_H
-#define _LIBS_UTILS_TIMERS_H
+#pragma once
 
 #include <stdint.h>
 #include <sys/types.h>
@@ -77,11 +73,11 @@
 static CONSTEXPR inline nsecs_t microseconds(nsecs_t v) { return us2ns(v); }
 
 enum {
-    SYSTEM_TIME_REALTIME = 0,  // system-wide realtime clock
-    SYSTEM_TIME_MONOTONIC = 1, // monotonic time since unspecified starting point
-    SYSTEM_TIME_PROCESS = 2,   // high-resolution per-process clock
-    SYSTEM_TIME_THREAD = 3,    // high-resolution per-thread clock
-    SYSTEM_TIME_BOOTTIME = 4   // same as SYSTEM_TIME_MONOTONIC, but including CPU suspend time
+    SYSTEM_TIME_REALTIME = 0,   // system-wide realtime clock
+    SYSTEM_TIME_MONOTONIC = 1,  // monotonic time since unspecified starting point
+    SYSTEM_TIME_PROCESS = 2,    // high-resolution per-process clock
+    SYSTEM_TIME_THREAD = 3,     // high-resolution per-thread clock
+    SYSTEM_TIME_BOOTTIME = 4,   // same as SYSTEM_TIME_MONOTONIC, but including CPU suspend time
 };
 
 // return the system-time according to the specified clock
@@ -104,5 +100,3 @@
 #ifdef __cplusplus
 } // extern "C"
 #endif
-
-#endif // _LIBS_UTILS_TIMERS_H