You need to install .NET 5 runtime. Assuming you're on Windows x64:
https://dotnet.microsoft.com/download/dotnet/thank-you/runtime-5.0.9-windows-x64-installer
I will make it self-contained for the next release, so you won't need to have .NET 5 installed.
By negative dates I mean BC dates. I'd like to output BC title history from Imperator cleanly (so a past Imperator ruler who ruled from 50.1.1 BC would begin his rule in -50.1.1 in the history files). CK3 doesn't support that, so best I can do is squash all BC rulers between 0-1(or 2?) AD in history files. Unless any CK3 modder has a better solution.
This problem is indeed irrelevant by 150 AD, but I want users to have the possibility to convert earlier (and get their rulers converted to children if it's TOO early ¯\_(ツ)_/¯).