On getting, the responseStart attribute MUST return as follows:

1. The time immediately after the user agent's HTTP parser receives the first byte of the response (e.g. frame header bytes for HTTP/2, or response status line for HTTP/1.x) from relevant application caches, or from local resources or from the server if the last non-redirected fetch of the resource passes the timing allow check algorithm.
2. zero, otherwise.

Specification