This library might help with implementation: GitHub - wanasit/chrono: A natural language date parser in Javascript · GitHub
That said I suspect it would be a challenge to get robust language support for this.
The current fully supported languages are
en,ja, andfr(deandptare partially supported). Other languages from v1 (nlandzh) are under development.
Also, another UX pattern to consider is automatically replacing date strings in the body of a post with Discourse date/time objects. Sort of like one-boxing but for dates. E.g. if I type “next monday at 2pm” in my post, it automatically converts that to 2021-08-17T18:00:00Z.