This is very much a front-end problem.
What makes the problem of showing input fields for dates and times difficult is that there exist so many different standards for entering this information. For instance, in the EU we commonly write
dd/mm/yyyy, while in america,
mm/dd/yyyy is used. The international ISO8601 standard is
yyyy-mm-dd which 'should resolve this. But of course, when introducing new standards...
You are definitely not the only developer looking for something like this. But as this is a very application-specific problem, and in any case is not something that is part of Phoenix' back-end, there indeed do not exist any Hex.pm packages providing such features.