This is just a hindsight, but I'd make two kinds of time period: "exact" period and "flexible" period. Define these as two different types, which can be convert

This is just a hindsight, but I'd make two kinds of time period: "exact" period and "flexible" period. Define these as two different types, which can be converted to each other but only explicitly. Functions like plus() can take both types (via method overload) but the user must be aware that their behavior is different.