diff --git a/usr.sbin/mtree/create.c b/usr.sbin/mtree/create.c
index f1b0313a746d..eee5037c2665 100644
--- a/usr.sbin/mtree/create.c
+++ b/usr.sbin/mtree/create.c
@@ -212,7 +212,7 @@ statf(int indent, FTSENT *p)
 		output(indent, &offset, "size=%jd",
 		    (intmax_t)p->fts_statp->st_size);
 	if (keys & F_TIME)
-		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);
 	if (keys & F_CKSUM && S_ISREG(p->fts_statp->st_mode)) {
diff --git a/usr.sbin/mtree/mtree.5 b/usr.sbin/mtree/mtree.5
index db5494306927..375cc7899690 100644
--- a/usr.sbin/mtree/mtree.5
+++ b/usr.sbin/mtree/mtree.5
@@ -200,7 +200,9 @@ The size, in bytes, of the file.
 .It Cm link
 The file the symbolic link is expected to reference.
 .It Cm time
-The last modification time of the file.
+The last modification time of the file, in seconds and nanoseconds.
+The value should include a period character and exactly nine digits
+after the period.
 .It Cm type
 The type of the file; may be set to any one of the following:
 .Pp
diff --git a/usr.sbin/mtree/mtree.8 b/usr.sbin/mtree/mtree.8
index 439eaab525ab..f0a93ed8cdad 100644
--- a/usr.sbin/mtree/mtree.8
+++ b/usr.sbin/mtree/mtree.8
@@ -233,7 +233,9 @@ The size, in bytes, of the file.
 .It Cm link
 The file the symbolic link is expected to reference.
 .It Cm time
-The last modification time of the file.
+The last modification time of the file, in seconds and nanoseconds.
+The value should include a period character and exactly nine digits
+after the period.
 .It Cm type
 The type of the file; may be set to any one of the following:
 .Pp
diff --git a/usr.sbin/mtree/spec.c b/usr.sbin/mtree/spec.c
index f2e7647a9a01..c7c6460de214 100644
--- a/usr.sbin/mtree/spec.c
+++ b/usr.sbin/mtree/spec.c
@@ -255,6 +255,8 @@ set(char *t, NODE *ip)
 		case F_TIME:
 			ip->st_mtimespec.tv_sec = strtoul(val, &ep, 10);
 			if (*ep == '.') {
+				/* Note: we require exactly nine
+				 * digits after the decimal point. */
 				val = ep + 1;
 				ip->st_mtimespec.tv_nsec
 				    = strtoul(val, &ep, 10);