Time of the current top-of-book quote, in seconds since epoch. This reflects the creation time of the best bid/ask order(s) currently resting at the top of book, not the time the message was generated. Because of this, the value is not monotonic: when the top of book changes to an order that was already resting in the book (for example after a trade), this timestamp may move backwards relative to the previous message. This is expected and does not indicate stale or dropped data.