When to use it
Use it as soon as you see a real chance of slipping. Name the deadline, say when you'll know for sure, and commit to confirming then. An early warning gives your manager time to adjust plans; a late one leaves them scrambling.
Example
You: “Heads-up: we're at risk of missing Friday. I'll know by Wednesday and confirm then.”
Manager: “Thanks for flagging. What would help?”
You: “If design can review Tuesday, we can still make it.”
In an email or Slack message
In Slack or email: “Flagging early: Friday's deadline is at risk because the data import is slow. I'll confirm by Wednesday. If it slips, the new date would be Tuesday.”