IsValidDate validates canary's issue date. Unlike Validate method, IsValidDate checks only existence of date and its
validity against provided time. Phrases defined in OMG specification are not checked. Returns nil if canary is valid.
Validate validates canary message. It looks for existence of mandatory phrases within the signed
message, a Bitcoin block and a date. It parses the date and checks if the message is valid for given time period.
Returns nil if canary is valid.