# Setting-up dead-letter queue retention in Amazon SQS
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For standard queues, the expiration of a message is always based on its original enqueue timestamp. When a message is moved to a dead-letter queue, the enqueue timestamp is unchanged. The `ApproximateAgeOfOldestMessage` metric indicates when the message moved to the dead-letter queue, *not* when the message was originally sent. For example, assume that a message spends 1 day in the original queue before it's moved to a dead-letter queue. If the dead-letter queue's retention period is 4 days, the message is deleted from the dead-letter queue after 3 days and the `ApproximateAgeOfOldestMessage` is 3 days. Thus, it is a best practice to always set the retention period of a dead-letter queue to be longer than the retention period of the original queue.

For FIFO queues, the enqueue timestamp resets when the message is moved to a dead-letter queue. The `ApproximateAgeOfOldestMessage` metric indicates when the message moved to the dead-letter queue. In the same example above, the message is deleted from the dead-letter queue after 4 days and the `ApproximateAgeOfOldestMessage` is 4 days.