“The user can choose between starting their new policy on the first day of employment, the first day of the fiscal year, on a specific date, or some number of days after their prior policy expires. If they choose the first day of the fiscal year, the user must specify when their company’s fiscal year starts. If they choose a specific date they must choose a date that is after the first business day of the next month and no later than December 31st of the year that month belongs to. If the user specified some number of months after their current policy expired the user must provide a policy number and the number of days no less than 1 and no more than 365.”

Type validation can help with some of that but at some point it becomes way easier to just use imperative validation for something like this. It turns out that validating things that are easy is easy no matter what you do, and validating complex rules that were written by people who think imperatively is almost impossible to do declaratively in a maintainable way.