Gets or sets the time at which every certificate of a chain must be within its validity period; null (the
default) for the time validation runs.
public DateTimeOffset? ValidationTime { get; set; }
RFC 5280 §6.1.1 (b) and §4.1.2.5. This is the current time of ISO 32000-2 §12.8.3.4.7: a time-stamping unit's path is validated at it, and a signer's path too unless a valid signature time-stamp token gives an earlier time (§12.8.3.4.8, see ValidationTime).