If I was hiring an agency to build a mobile app, and one of them came back with "195 days," I would have pretty high confidence that they've never built anythin

If I was hiring an agency to build a mobile app, and one of them came back with "195 days," I would have pretty high confidence that they've never built anything of noteworthy complexity before, and that they think that software projects can be planned to the day months in advance. On the other hand, for operational promises (e.g., a 24-hour SLA) means something more specific than a 1-day SLA: 24 hours is clear, whereas anything based on days requires definitions (Literally the next day? After one full calendar day has passed? So, no, smaller units are not more credible... smaller units make a more specific claim which in some cases is more credible, but in some cases is not credible.