Once a developer has done a project, he can give a good estimate of how long it would take for him to do that project again!
Really, it's easy for developers to estimate time much the same way plumbers, carpenters, auto body shop workers, general contractors, dentists, etc. do: When a developer has done a very similar project more than 10 times recently, they can give a good estimate for how long it will take to do the project once more.
This situation is quite general in engineering and other work of wide variety.
Similarly, in general, when some project is being done for the first time, with no prior experience with comparable projects, then time estimates are tough to give.
In particular, for 'developers', now they work heavily with 'parts, pieces, and tools' from others, and working with these for the first time encounters standard problems: (A) How to use the parts, etc., needs good documentation rarely available. (B) Newer parts, etc. commonly have strange behavior or actual bugs that have to be encountered, diagnosed, and worked around. So a developer does not know how much time will be spent mud wrestling with (A) and (B).
Except that no two projects are ever really the same. Even with a very similar project you'll have different people, different systems to interface with and different scope.
Plus, any developer worth their salt will have built libraries as they went (or documented the existing ones), so the next similar project will take much, much less time.
Really, it's easy for developers to estimate time much the same way plumbers, carpenters, auto body shop workers, general contractors, dentists, etc. do: When a developer has done a very similar project more than 10 times recently, they can give a good estimate for how long it will take to do the project once more.
This situation is quite general in engineering and other work of wide variety.
Similarly, in general, when some project is being done for the first time, with no prior experience with comparable projects, then time estimates are tough to give.
In particular, for 'developers', now they work heavily with 'parts, pieces, and tools' from others, and working with these for the first time encounters standard problems: (A) How to use the parts, etc., needs good documentation rarely available. (B) Newer parts, etc. commonly have strange behavior or actual bugs that have to be encountered, diagnosed, and worked around. So a developer does not know how much time will be spent mud wrestling with (A) and (B).