internal/timeseries: avoid storing numbers as durations

... even if it does avoid a cast. advance isn't a duration and shouldn't be typed that way.

Change-Id: I6328188e74010d6c57adf5b26beede57b3bc7e33
GitHub-Last-Rev: 281097649e15c343e72cc51ecbeda31e81f9b6b3
GitHub-Pull-Request: golang/net#29
Reviewed-on: https://go-review.googlesource.com/c/net/+/161297
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

internal/timeseries/timeseries.go

@@ -403,9 +403,9 @@ func (ts *timeSeries) extract(l *tsLevel, start, finish time.Time, num int, resu
 
 	// Where should scanning start?
 	if dstStart.After(srcStart) {
-		advance := dstStart.Sub(srcStart) / srcInterval
-		srcIndex += int(advance)
-		srcStart = srcStart.Add(advance * srcInterval)
+		advance := int(dstStart.Sub(srcStart) / srcInterval)
+		srcIndex += advance
+		srcStart = srcStart.Add(time.Duration(advance) * srcInterval)
 	}
 
 	// The i'th value is computed as show below.