I can have this major feature done for you, but I'm not sure if it can be done in 2 months, so I say 3 to be conservative.
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.
If you can't keep promises, and don't want to bullshit, then talk more about what the product already does.