diff --git a/libutils/Timers.cpp b/libutils/Timers.cpp
index 5293cd2..a431e92 100644
--- a/libutils/Timers.cpp
+++ b/libutils/Timers.cpp
@@ -34,7 +34,7 @@
 
 nsecs_t systemTime(int clock)
 {
-#if defined(HAVE_POSIX_CLOCKS)
+#if defined(HAVE_ANDROID_OS)
     static const clockid_t clocks[] = {
             CLOCK_REALTIME,
             CLOCK_MONOTONIC,
@@ -47,7 +47,9 @@
     clock_gettime(clocks[clock], &t);
     return nsecs_t(t.tv_sec)*1000000000LL + t.tv_nsec;
 #else
-    // we don't support the clocks here.
+    // Clock support varies widely across hosts. Mac OS doesn't support
+    // posix clocks, older glibcs don't support CLOCK_BOOTTIME and Windows
+    // is windows.
     struct timeval t;
     t.tv_sec = t.tv_usec = 0;
     gettimeofday(&t, NULL);