What does nginx's $time_local logging variable mean, specifically?

I'm reading some nginx logs for some rather long-lasting requests (up to 10 seconds). In the log format, we're using the $time_local variable to record the time. But does this mean the time when the request started or when it finished? For a long-running request, these are quite distinct times, and I can't find anything in the ngx_http_log_module documentation to explain this.

[kazeburo]

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