design.uml

+-----------------------+
|  ReferencePeriod    |
+-----------------------+
| - start: DateTime   |
| - end: DateTime     |
+-----------------------+
| + __init__(start, end) |
+-----------------------+

+-----------------------+
|  ValidityPeriod    |
+-----------------------+
| - start: DateTime   |
| - end: DateTime     |
+-----------------------+
| + __init__(start, end) |
| + is_valid(reference_period_end) |
+-----------------------+


@startuml
skinparam class {
    FontName Arial
    FontSize 14
    ArrowColor Black
}

class DateTime {
    + iso_datetime : DateTime
    + iso_string : str

    constructor (value : str)
    method now() : DateTime
    method create_datetime_years_from_now(years : int) : DateTime
    static method same_day(first : DateTime, second : DateTime) : bool
    method ==(other : object) : bool
    method <(other : DateTime) : bool
    method <=(other : DateTime) : bool
    method >(other : DateTime) : bool
    method >=(other : DateTime) : bool
    method __repr__() : str
    method __str__() : str
    property date
    property year : int
    property month : int
    property day : int
    property time : str
}

@enduml