The planning fallacy makes a forecast too optimistic because the current project is easy to picture in its clean, intended order.
A team estimating a website migration can argue over its ideal schedule for hours.
A better starting point is the actual duration of 5 similar migrations, including the ordinary testing, approval, and integration delays that plans tend to soften.
That outside view may push the date later, but it starts from what work has really taken before.
Adjust only after that. The question is not how perfect this plan looks; it is what usually happens to plans like it.