Company wanted to port an OS to 32-bit (yes I know that dates me). I spent a day, reviewed the code (500 modules), wrote a report - two manyears. Project deferred (ashcanned).
A month later my peer, a senior Engineer with the Company, thought "We should run 32-bit! How long can it take?", checked out the source and started changing files. Gradually people noticed what he was doing, resources got added under the table, finally a year later he had something working. Lots of kudos, smiles, what a hero!
I showed him the report I filed a year ago - and he said "yeah, that's about what I had to do. I'm glad I didn't see that to begin with, I would never have started".
Two man-years can easily become one calendar year, if 2-3 people are working fulltime on the project.
Also, "two man-years" isn't a complete estimate using my approach. "One to two man-years" would be a complete estimate, providing a range. Sometimes you really do come in near the bottom of the range, and maybe a month into it if you re-estimated you would have come up with a lower range.
A month later my peer, a senior Engineer with the Company, thought "We should run 32-bit! How long can it take?", checked out the source and started changing files. Gradually people noticed what he was doing, resources got added under the table, finally a year later he had something working. Lots of kudos, smiles, what a hero!
I showed him the report I filed a year ago - and he said "yeah, that's about what I had to do. I'm glad I didn't see that to begin with, I would never have started".