diff --git a/Validity.md b/Validity.md
index 316c96d..41c4279 100644
--- a/Validity.md
+++ b/Validity.md
@@ -42,7 +42,12 @@ Please be careful about the format,  there are only `+*d` (for days) and `+*h` (
 
 
 ## If you use the absolute format for `--valid-to  "2022-04-01T08:10:33Z"`,   the cert will NOT be renewed automatically when it expires.  
-## If you want the cert to be renewed automatically, please use the relative format `--valid-to  +20d`(the cert will be renewed every 20 days).   Of course, if you don't use `--valid-to` parameter at all, the cert will be renewed every `60 days` as before.
+
+## If you want the cert to be renewed automatically, please use the relative format:
+  1. `--valid-to  +20d`(the cert will be renewed every 19 days).   If the lifetime is longer than one day, it will renew at one day before.
+  2. `--valid-to  +11h`(the cert will be renewed every 10 hours).  If the lifttime is less than 24 hourst, it will renew at one hour before.
+
+## Of course, if you don't use `--valid-to` parameter at all, the cert will be renewed every `60 days` as before.