Updated Validity (markdown)

Validity.md

@@ -23,6 +23,23 @@ Please be careful about the date time format,  it Must be exact format used abov
 
 
 
+You can also use a relative date time format:
+
+```
+# This cert will only be valid for `10` days.
+
+acme.sh  --issue  -d example.com --dns dns_cf   --valid-to  "+10d"
+
+
+# This cert will be valid for `30` hours.
+
+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.
+
+