Their work starts when yours lands. You now know yours will not land. The draft you keep softening spends a paragraph on why, a paragraph on how bad you feel, and a last line that says you are "hoping to get it over soon."
They cannot schedule around hope. They can schedule around a date.
Lead with the date and the ask
"The API contract will be Thursday 18:00 UTC, not Tuesday. You can keep Friday for integration if that still holds. I need you to tell me if Friday is now impossible, and if so which of two cuts you want."
Two or three sentences. That is a note that respects their time. They know what moved, what is still true, and what you need back.
The why can sit lower, in four lines, as a record: what slipped, what you tried, what you are not asking them to fix. A peer is not your manager. They do not need the career version of the story. They need to move their own block.
Give them a choice, not a problem
"Sorry, it will be late, let me know how you want to handle it" hands them an open problem they understand less than you do. The predictable replies are silence, or a meeting.
Close the list:
- They wait. Name the new date.
- They start against a stub. Name what the stub will contain.
- They cut their piece. Name the piece.
Set a date you will keep. A second slip on the same promise costs more than the first, because the first taught them your dates are opening bids.
If you do not know the date, say when you will. That is still a deadline — on information, not on the work.
Do not turn the follow-up into a nudge campaign
One clear note beats three "just checking you saw this." If you need an answer, say when you will assume a default. Following up without nagging is the same discipline here as anywhere else: add a fact, or add a tighter default, or stay quiet.
A private nickname you have for them does not belong in the note. Use the name they use on the team.
Send it before their day starts on a dead promise
If they have a RelayLink account, you need an accepted contact for a new thread. If they do not, they get email and a page; they can reply from the link. You approve the wording. An assistant can assemble the options from the session where you already knew you were late. It cannot send the apology for you, and it should not.
Write it as soon as the old date is no longer true. The kind version is early and specific. The kind-sounding version is late and vague.
If you already talked the slip through with an assistant, connect it and have it put the new date in the first sentence.