diff --git a/src/disk_interface.cc b/src/disk_interface.cc
index 4a5ea34..47a02cc 100644
--- a/src/disk_interface.cc
+++ b/src/disk_interface.cc
@@ -160,19 +160,13 @@
   // that it doesn't exist.
   if (st.st_mtime == 0)
     return 1;
-#if defined(__APPLE__) && !defined(_POSIX_C_SOURCE)
+#if defined(_AIX)
+  return (int64_t)st.st_mtime * 1000000000LL + st.st_mtime_n;
+#elif defined(__APPLE__)
   return ((int64_t)st.st_mtimespec.tv_sec * 1000000000LL +
           st.st_mtimespec.tv_nsec);
-#elif (_POSIX_C_SOURCE >= 200809L || _XOPEN_SOURCE >= 700 || defined(_BSD_SOURCE) || defined(_SVID_SOURCE) || \
-       defined(__BIONIC__) || (defined (__SVR4) && defined (__sun)) || defined(__FreeBSD__))
-  // For glibc, see "Timestamp files" in the Notes of http://www.kernel.org/doc/man-pages/online/pages/man2/stat.2.html
-  // newlib, uClibc and musl follow the kernel (or Cygwin) headers and define the right macro values above.
-  // For bsd, see https://github.com/freebsd/freebsd/blob/master/sys/sys/stat.h and similar
-  // For bionic, C and POSIX API is always enabled.
-  // For solaris, see https://docs.oracle.com/cd/E88353_01/html/E37841/stat-2.html.
+#elif defined(st_mtime) // A macro, so we're likely on modern POSIX.
   return (int64_t)st.st_mtim.tv_sec * 1000000000LL + st.st_mtim.tv_nsec;
-#elif defined(_AIX)
-  return (int64_t)st.st_mtime * 1000000000LL + st.st_mtime_n;
 #else
   return (int64_t)st.st_mtime * 1000000000LL + st.st_mtimensec;
 #endif