Updated Validity (markdown)

Validity.md

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-validity
+The ACME protocol supported the `NotBefore` and `NotAfter` fields of the cert.
+And some of the CAs supported this feature. (The Letsencrypt CA doesn't support it for now)
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+There are 2 command options to use:
+
+1. The `--valid-to <date time>` option, which is for `NotAfter` field.
+2. The `--valid-from <date time>` option, which is for `NotBeofre` field.
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+Usage: 
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+### 1.  Set the lifetime of the cert:
+
+```
+acme.sh --issue  -d  example.com  -dns dns_cf    --valid-to  "2022-04-01T08:10:33Z"
+```
+
+The value of `--valid-to` is an absolute date time in the future.   The issued cert will expire on that time(`NotAfter`).
+
+Please be careful about the date time format,  it Must be exact format used above.
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