Jeff Madsen has a post on his Medium blog sharing some of his thoughts about immutable DateTime types, what the difference is between mutable and immutable and "why you should care".

I’m going to show you the difference between the two using two popular Php DateTime libraries? - Carbon and Chronos, and then demonstrate the danger of using the mutable one of those.

You have probably used Carbon? - ?it is a wonderful library put together by Brian Nesbitt that takes all the pain out of working with dates. It has got one “short-coming”, if you will?- ?it is built on top of the DateTime object.

He gives an example of why this is a problem with Carbon (mutable) and how it's handled differently in Chronos (immutable). He makes the point that, unless your date values are immutable, you don't have any idea of they've changed elsewhere in the processing. He gives a more real-world example of working with immutable objects with a "user" model class and the "name" properties attached to it.