I was curious as to the internals of the Date struct, but I simply can't find it on github. I searched primarily in the main swift repo and the swift-foundation repo, but the only think I turned up was a discussion in Comparable...
GitHub's code search is pretty good, if you're signed in. In that case, go to the repo you want to search and search for specific terms. In this case I searched for "struct Date" (with the quotes) to find the definition.