doc: recommend in-window X509Certificate validity check

Per review, tell callers to test that the current time is inside the
validity window, which rejects an invalid bound, instead of testing
whether it is outside the window, which an invalid bound passes.

Signed-off-by: Sarthak Ahuja <39986642+SarthakAhuja@users.noreply.github.com>

Author: SarthakAhuja

doc/api/crypto.md

@@ -3534,8 +3534,12 @@ The date/time from which this certificate is valid, encapsulated in a `Date` obj
 
 If the certificate's `notBefore` time cannot be parsed, this is an invalid
 `Date` (its `getTime()` method returns `NaN`). Every relational comparison
-with an invalid `Date` evaluates to `false`, so callers should reject an
-invalid `Date` before comparing it with other dates.
+with an invalid `Date` evaluates to `false`. To check whether a certificate
+is currently valid, test that the current time is inside the validity window
+(`now >= x509.validFromDate && now <= x509.validToDate`), which rejects an
+invalid bound. Do not test whether it is outside the window
+(`now < x509.validFromDate || now > x509.validToDate`): with an invalid
+bound, that test is `false` and the certificate appears valid.
 
 ### `x509.validTo`
 
@@ -3564,8 +3568,12 @@ The date/time until which this certificate is valid, encapsulated in a `Date` ob
 
 If the certificate's `notAfter` time cannot be parsed, this is an invalid
 `Date` (its `getTime()` method returns `NaN`). Every relational comparison
-with an invalid `Date` evaluates to `false`, so callers should reject an
-invalid `Date` before comparing it with other dates.
+with an invalid `Date` evaluates to `false`. To check whether a certificate
+is currently valid, test that the current time is inside the validity window
+(`now >= x509.validFromDate && now <= x509.validToDate`), which rejects an
+invalid bound. Do not test whether it is outside the window
+(`now < x509.validFromDate || now > x509.validToDate`): with an invalid
+bound, that test is `false` and the certificate appears valid.
 
 ### `x509.signature`