On this page: http://blogs.msdn.com/b/mike/archive/2008/11/13/time-vs-time-taken-fields-in-iis-logging.aspx
it says:
The Time field is quite straightforward: it specifies when the log entry was created. Note that this is not always the same as when the log entry actually gets written to the log, as buffering can occur for some request/response scenarios.
Therefore, you are correct in thinking that the time most closely corresponds to the time that the request finished. That same page goes on to clarify:
If you wanted to calculate the approximate Start Time of a request, you would
subtract the Time-Taken value from the Time value.