internal: use time.Since instead of time.Now().Sub

internal:  use time.Since instead of time.Now().Sub

Change-Id: I536c7fad84a63e96658c6930a5a77fd70edca33c
Reviewed-on: https://go-review.googlesource.com/c/go/+/436718
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>