This is the golden rule of software dev too - promise features or promise a release date, but never both.
I agree and it's such a golden rule that no one follows or cares about even the slightest. Features and deadlines are to SE like velocities and positions are to QM.
This idea more thoroughly described here: https://news.ycombinator.com/item?id=21212087
If you can't keep promises, and don't want to bullshit, then talk more about what the product already does.
OR
I can have many small sub-features that relate to this major feature done in 2 months, but I might know exactly which features make the cut in those 2 months.
Seems congruous to me