negative duration values

[plehegar]

From https://lists.w3.org/Archives/Public/public-web-perf/2015Jan/0049.html
[[
In working on the implementation of the User Timing spec in Gecko, it was
brought up about the potential of durations to return negative values.
Specifically it was unclear from the spec whether things like this should
be allowed:

performance.mark('A');
performance.mark('B');

performance.measure('delta', 'B', 'A');

assert( performance.getEntriesByName('delta')[0].duration < 0 ); // true

Chrome allows this interaction, enabling negative duration values for
measures. Is this intended, and if so, should we clarify the spec that
negative durations are allowed for performance measurements specifically?
]]

[igrigorik]

I think negative values are reasonable, and AFAIK, IE allows negative values as well.

[toddreifsteck]

Ilya is correct. A quick test during the call when we discussed this issue demonstrated that many browsers allow negatives. I believe we should update the spec with this language.

[toddreifsteck]

@phlegar Please let us know if you have feedback on the merged update. Thanks!