diff --git a/flist.c b/flist.c
index 7bf3030e5..77dfc6224 100644
--- a/flist.c
+++ b/flist.c
@@ -132,6 +132,20 @@ static int64 tmp_dev = -1, tmp_ino;
 #endif
 static char tmp_sum[MAX_DIGEST_LEN];
 
+#ifdef ST_MTIME_NSEC
+/* Return st_mtim nsec if it is in the wire-valid range, else 0.  A pre-1970
+ * mtime with a sub-second part yields a negative tv_nsec on some platforms
+ * (e.g. cygwin), which a protocol-31 peer rejects as an out-of-range value. */
+static inline uint32 wire_mtime_nsec_from_stat(const STRUCT_STAT *stp)
+{
+	unsigned long nsec = (unsigned long)stp->ST_MTIME_NSEC;
+
+	if (nsec > MAX_WIRE_NSEC)
+		return 0;
+	return (uint32)nsec;
+}
+#endif
+
 static char empty_sum[MAX_DIGEST_LEN];
 static int flist_count_offset; /* for --delete --progress */
 static int show_filelist_progress;
@@ -1401,7 +1415,7 @@ struct file_struct *make_file(const char *fname, struct file_list *flist,
 	}
 
 #ifdef ST_MTIME_NSEC
-	if (st.ST_MTIME_NSEC && protocol_version >= 31)
+	if (wire_mtime_nsec_from_stat(&st) && protocol_version >= 31)
 		extra_len += EXTRA_LEN;
 #endif
 #if SIZEOF_CAPITAL_OFF_T >= 8
@@ -1457,9 +1471,13 @@ struct file_struct *make_file(const char *fname, struct file_list *flist,
 	file->flags = flags;
 	file->modtime = st.st_mtime;
 #ifdef ST_MTIME_NSEC
-	if (st.ST_MTIME_NSEC && protocol_version >= 31) {
-		file->flags |= FLAG_MOD_NSEC;
-		F_MOD_NSEC(file) = st.ST_MTIME_NSEC;
+	{
+		uint32 nsec = wire_mtime_nsec_from_stat(&st);
+
+		if (nsec && protocol_version >= 31) {
+			file->flags |= FLAG_MOD_NSEC;
+			F_MOD_NSEC(file) = nsec;
+		}
 	}
 #endif
 	file->len32 = (uint32)st.st_size;
diff --git a/rsync.h b/rsync.h
index 479ac4848..31ceee558 100644
--- a/rsync.h
+++ b/rsync.h
@@ -173,6 +173,8 @@
 #define MAX_BASIS_DIRS 20
 #define MAX_SERVER_ARGS (MAX_BASIS_DIRS*2 + 100)
 
+#define MAX_WIRE_NSEC          999999999
+
 #define COMPARE_DEST 1
 #define COPY_DEST 2
 #define LINK_DEST 3