TimeWindow()
An interval on the time-line; not a single instantaneous point.
There is no such thing as exact time in networked systems due to the underlying physics involved and other issues such as network latency. The best that can be approximated is "fuzzy time" or an instant of time which has margin of tolerance around it. This is what TimeWindow represents. Time windows can be open-ended (i.e. specify only one of fromTime and untilTime) or they can be fully bounded.
WireTransaction has an optional time-window property, which if specified, restricts the validity of the transaction to that time-interval as the Consensus Service will not sign it if it's received outside of this window.