Suggest Time.Sub instead of time.Now+time.Since

`time.Since()` executed `time.Now()` under the hood.
If we need `time.Time` as well as `time.Duration` (since), we can do this optimization:
```go
elapsed := time.Since(oldTime)
oldTime = time.Now()

// =>

newTime := time.Now()
elapsed := newTime.Sub(oldTime)
oldTime = newTime
```