If we add a leap second at the end of 2025, nothing in 2024 gets rewritten. Only the future meaning of pointer expressions like "12 pm on January 2nd 2025" change their value. When I want exactly 48 hours after 12 pm Dec 31, I use a leap second independent time representation. But since usually I want the same thing everyone calls 12 pm Jan 2, I usually use a representation that gives me that.

And I, among many, take meticulous care to do my date math (for a bank core system) only in ways that naturally support leap seconds.