Software time estimation

Software project time and effort estimation is difficult. One of the difficulties arise from writing low quality components. These components incur technical debt (or they might just be a mess), break the system at a later time when no one remembers their details, and require frequent revisits. Poor components are usually difficult to understand too. All this results in a lot of unbudgeted effort and delays. Even if you estimated the time for completing the messy version of the component, you would not be able to take into account the additional cost required to get to a clean/stable version.

The corollary is that you have to focus on quality before you spend any time on schedule estimation. To achieve acceptable quality, write unit tests, do code reviews (and at least rudimentary algorithm documentation) and use static analysis tools. Only after you have unit tests, documentation, code reviews and static analysis in place, you might try to guess when the project finishes.