diff --git a/Validity.md b/Validity.md
index 16cb99f..8a35e06 100644
--- a/Validity.md
+++ b/Validity.md
@@ -37,7 +37,14 @@ acme.sh  --issue  -d example.com --dns dns_cf   --valid-to  "+30h"
 
 ```
 
-Please be careful about the format,  there are only `+*d` (for days) and `+*h` (for hours) supported for now. Any other format will result errors.
+Please be careful about the format,  there are only `+*d` (for days) and `+*h` (for hours) supported for now. Any other format will not be accepted.
+
+
+## If the lifetime is measured in hours,  you need to change the default crontab to run `acme.sh` every an hour:
+
+```
+0 * * * * "/root/.acme.sh"/acme.sh --cron --home "/root/.acme.sh" > /dev/null
+```