The effective date is the date after which the X.509 certificate is considered valid.

The X509Certificate.GetEffectiveDateString method returns a string that shows the date formatted in Short Date Pattern followed by the time formatted in Long Time Pattern. The date and time are formatted using the current culture and time zone.

Namespace: System.Security.Cryptography.X509Certificates
Assembly: mscorlib (in mscorlib.dll)
Assembly Versions: 1.0.5000.0, 2.0.0.0, 4.0.0.0