Correctly format the nanoseconds by padding out to 9 digits.
Document this with text from the FreeBSD manpage.

Without this change a file modified on the first nanosecond of a second
(N.000000001) would be reported to have been modified at precicely 100ms
into the second (N.100000000).

Index: mtree.8
===================================================================
--- mtree.8	(revision 240766)
+++ mtree.8	(revision 240767)
@@ -492,7 +492,10 @@
 These may be specified without leading or trailing commas, but will be
 stored internally with them.
 .It Sy time
-The last modification time of the file.
+The last modification time of the file,
+in second and nanoseconds.
+The value should include a period character and exactly nine digits after
+the period.
 .It Sy type
 The type of the file; may be set to any one of the following:
 .Pp
Index: create.c
===================================================================
--- create.c	(revision 240766)
+++ create.c	(revision 240767)
@@ -215,11 +215,11 @@
 		    (long long)p->fts_statp->st_size);
 	if (keys & F_TIME)
 #if defined(BSD4_4) && !defined(HAVE_NBTOOL_CONFIG_H)
-		output(indent, &offset, "time=%ld.%ld",
+		output(indent, &offset, "time=%ld.%09ld",
 		    (long)p->fts_statp->st_mtimespec.tv_sec,
 		    p->fts_statp->st_mtimespec.tv_nsec);
 #else
-		output(indent, &offset, "time=%ld.%ld",
+		output(indent, &offset, "time=%ld.%09ld",
 		    (long)p->fts_statp->st_mtime, (long)0);
 #endif
 	if (keys & F_CKSUM && S_ISREG(p->fts_statp->st_mode)) {
Index: spec.c
===================================================================
--- spec.c	(revision 240766)
+++ spec.c	(revision 240767)
@@ -358,7 +358,7 @@
 		if (MATCHFLAG(F_SIZE))
 			appendfield(pathlast, "size=%lld", (long long)cur->st_size);
 		if (MATCHFLAG(F_TIME))
-			appendfield(pathlast, "time=%lld.%ld",
+			appendfield(pathlast, "time=%lld.%09ld",
 			    (long long)cur->st_mtimespec.tv_sec,
 			    cur->st_mtimespec.tv_nsec);
 		if (MATCHFLAG(F_CKSUM))