Contract expiry time vs sell time
Expiry time is when the option expires.
Sell time is when the option is sold by the client - he can sell it before expiry (at the current market price), or after expiry (when it is worth either 0 or full payout).
Put it simply, sell time can occur prior, at or after expiry. Whereas, expiry time is the predefined expiry/maturity time of a contract.