Index: head/usr.bin/diff/diffreg.c
===================================================================
--- head/usr.bin/diff/diffreg.c
+++ head/usr.bin/diff/diffreg.c
@@ -94,12 +94,6 @@
 
 #define _PATH_PR "/usr/bin/pr"
 
-#ifdef ST_MTIM_NSEC
-# define TIMESPEC_NS(timespec) ((timespec).ST_MTIM_NSEC)
-#else
-# define TIMESPEC_NS(timespec) 0
-#endif
-
 /*
  * diff - compare two files.
  */
@@ -1615,14 +1609,10 @@
 	char end1[10];
 	char end2[10];
 	struct tm tm1, tm2, *tm_ptr1, *tm_ptr2;
-	int nsec1 = TIMESPEC_NS (stb1.st_mtime);
-	int nsec2 = TIMESPEC_NS (stb2.st_mtime);
+	int nsec1 = stb1.st_mtim.tv_nsec;
+	int nsec2 = stb2.st_mtim.tv_nsec;
 
-#ifdef ST_MTIM_NSEC
-		time_format = "%Y-%m-%d %H:%M:%S.%N";
-#else
-		time_format = "%Y-%m-%d %H:%M:%S";
-#endif
+	time_format = "%Y-%m-%d %H:%M:%S";
 
 	if (cflag)
 		time_format = "%c";
Index: head/usr.bin/diff/tests/Makefile
===================================================================
--- head/usr.bin/diff/tests/Makefile
+++ head/usr.bin/diff/tests/Makefile
@@ -20,7 +20,8 @@
 	unified_p.out \
 	unified_c9999.out \
 	unified_9999.out \
-	header.out
+	header.out \
+	header_ns.out
 
 NETBSD_ATF_TESTS_SH+=	netbsd_diff_test
 
Index: head/usr.bin/diff/tests/diff_test.sh
===================================================================
--- head/usr.bin/diff/tests/diff_test.sh
+++ head/usr.bin/diff/tests/diff_test.sh
@@ -3,6 +3,7 @@
 atf_test_case simple
 atf_test_case unified
 atf_test_case header
+atf_test_case header_ns
 
 simple_body()
 {
@@ -58,9 +59,21 @@
 		diff -u empty hello
 }
 
+header_ns_body()
+{
+	export TZ=UTC
+	: > empty
+	echo hello > hello
+	touch -d 2015-04-03T01:02:03.123456789 empty
+	touch -d 2016-12-22T11:22:33.987654321 hello
+	atf_check -o "file:$(atf_get_srcdir)/header_ns.out" -s eq:1 \
+		diff -u empty hello
+}
+
 atf_init_test_cases()
 {
 	atf_add_test_case simple
 	atf_add_test_case unified
 	atf_add_test_case header
+	atf_add_test_case header_ns
 }
Index: head/usr.bin/diff/tests/header_ns.out
===================================================================
--- head/usr.bin/diff/tests/header_ns.out
+++ head/usr.bin/diff/tests/header_ns.out
@@ -0,0 +1,4 @@
+--- empty	2015-04-03 01:02:03.123456789 +0000
++++ hello	2016-12-22 11:22:33.987654321 +0000
+@@ -0,0 +1 @@
++hello