Backoff policy with no external fields for counter.

backoff.go

var backoffPolicy = []int64{0, 10, 30, 60, 300, 300, 900, 900, 900, 1800}

func BackoffTime(t int64, base int64) int64 {
	r := t % 10
	// use base var to avoid creating another variable
	// round base to upper value multiple of ten seconds
	// and add backoff time
	base += 10 - base%10 + backoffPolicy[r] + r
	if r != 9 {
		base++
	}
	return base
}