Currently 0 time is selected as the first timestamp found in the trace. However this will skew results the more that there's a gap between tracing started and navigation started.

Ideally we should find the navigationStart in the trace if a timeOrigin is not provided in the options.
Doing that is fairly straightforward:

https://github.com/GoogleChrome/lighthouse/blob/master/lighthouse-core/gather/computed/trace-of-tab.js#L53-L67