Duper certainly won't outperform the native JSON implementation (and it likely never will), though I do think benchmarks would be a great addition. Bundle size and binary representation are definitely things I'll keep in mind!
The config file transpiration to JSON idea is quite interesting. It's pretty similar to how I'm already defining the TextMate grammar used by the website's syntax highlighter, so I'll certainly try to incorporate that into the tooling.
My reasoning is that they are normally transmitted as strings in JSON, and you could use an identifier like DateTime("2025-11-02T02:33:00Z") if you need to be explicit.
Making them part of the language would increase the complexity of parsers - how would you validate that a date is actually valid? It's doable (YAML and TOML do it, after all) but requires extra steps.