How can I choose the latest date in a child-object's records for a table in the parent-object?

I followed the instructions from "How do I find the most recent date from a connected object?", but the dates I wind up with (after converting to a number in the child object, then going to the parent object and using Max to find the new one, then converting back from number to date) are WAY off. If I use "hours" as a date format, I get numbers about 50,000 years in the future. If I use "days" as the date format, all the resulting dates are from 1970, when they should be in Spring 2021. 

Hi - I have used that approach in the past - but I have a working example where I just use a formula in the child object like this:

And then use a Max field against this - seems to work as expected.