I often hear it used this way. I'd call the usage casual rather than formal. Your contraction needs an appostrophe: No one's supposed to arrive etc.
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He can come at 5pm or 8pm. With him, you never know. He's always late.
In conversation, we sometimes have the habit of using too many words. When writing, it's ideal to use the fewest words to communicate the thought. Generally, this allows the reader to take in the information faster and clearer.
Hope