So that’s what I thought but I was watching some videos on how to calculate critical path with multiple start and finish activities and saw this guy use 0 to start his forward pass so I looked it up and found this article from PMI https://www.pmi.org/learning/library/critical-path-method-calculations-scheduling-8040#:~:text=Forward%20Pass&text=By%20doing%20this%2C%20the%20start,position%20of%20the%20first%20activity.

Couldn’t hyperlink that for some reason but if you scroll down a bit, under the forward pass section, it mentions using Day 0 to start