@bcmills , @ianlancetaylor can you give this a look?

x/time/rate is more @Sajmani...

I think this is work as designed. It's covered by the test TestLimiterJumpBackwards (https://github.com/golang/time/blob/f3bd1da661afd6357b957af27c50ccdb248b7dd3/rate/rate_test.go#L124-L138). On line 127, it jumps backward from t1 to t0. And then on line 131, it jumps forward from t0 to t1, and it got a new token.

In your demo, every time it jumps from t1 to t0, it gets two new tokens. That's why the available tokens never reduce to 0.

By the way, @tentree please change your comment to put your code into a fenced code block:

```go
package main

import (
	"fmt"
	"time"

	"golang.org/x/time/rate"
)

func main() {
	limiter := rate.NewLimiter(1, 10)

	t0 := time.Now()
	t1 := time.Now().Add(-time.Second * 2)

	t := t0
	for limiter.AllowN(t, 1) {
		if t == t0 {
			t = t1
		} else {
			t = t0
		}
	}

	fmt.Println("exit")
}
```

I think this is work as designed. It's covered by the test TestLimiterJumpBackwards (https://github.com/golang/time/blob/f3bd1da661afd6357b957af27c50ccdb248b7dd3/rate/rate_test.go#L124-L138). On line 127, it jumps backward from t1 to t0. And then on line 131, it jumps forward from t0 to t1, and it got a new token.

In your demo, every time it jumps from t1 to t0, it gets two new tokens. That's why the available tokens never reduce to 0.

By the way, @tentree please change your comment to put your code into a fenced code block:

package main

import (
	"fmt"
	"time"

	"golang.org/x/time/rate"
)

func main() {
	limiter := rate.NewLimiter(1, 10)

	t0 := time.Now()
	t1 := time.Now().Add(-time.Second * 2)

	t := t0
	for limiter.AllowN(t, 1) {
		if t == t0 {
			t = t1
		} else {
			t = t0
		}
	}

	fmt.Println("exit")
}

since t1 to t0 could got new tokens, i think t0 back to t1 also could reduce tokens. This is what I'm confused about.

@tentree Please don't quote an entire comment unnecessarily (edit your comment to remove it). Jumping backward won't reduce the tokens. It's also covered by that same test (line 127, jump back to t0, two tokens remain).

@ZekeLu thanks , i may understand the designe of limiter. back in the past just like fixed clock. the func with N（like AllowN） is just for testing easily, Is my understanding biased？

In my opinion, the time.Time parameter in the *N funcs is just for testing. Regarding whether the tokens should be dropped when it jumps backwards, there are some interesting comments here: https://go-review.googlesource.com/c/time/+/16672/2..6/rate/rate.go#b65

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