BUILD is called after the constructor, which makes it handy to verify state but not necessarily useful to format incoming arguments.

BUILDARGS would let you modify incoming arguments before the constructor is called, which makes it a better fit for this case. Your attribute is read-only, so this could work.

But... if you're hungry for static typing, why would you stop after promising "this is a string"? If you create a subtype for ISO8601 strings, you can promise "this is a string and it has X format". Even better, you're doing that in a way that's immediately and trivially portable to other attributes.

I rather doubt the regex below will work for you, but I hope it will get the point across:

#define the type
subtype 'iso8601',
    as 'Str',
    where { /\d{4}-\d{2}-\d{2}/ },
    message { "Not a valid ISO8601 string ($_)" };

#provide a coercion
coerce 'iso8601',
    from 'Str',
    via { _format_as_iso8601 $_ };

#tell moose to coerce the value
has startdate  => (is => 'ro', isa => 'iso8601', required => 1, coerce => 1);