[raspasov]

Dates and times are values by definition, not mutable objects that can be changed via methods. The year 2017 is 2017. It represent a moment in time, or more precisely a period of time between two points. The year 2017 does not “become” 2018 when you .add one year to it. Fully immutable objects are also easier and simpler to compare since they are, again, values. 2017 is 2017, always. Not using proper immutability leads to an “object identity” crisis where everything is anything. An infinite amount of years 2017 are allowed to exist by default(!) and they are not easily comparable or require specific library APIs to allow that.