How does making JavaScript Temporal date-and-time values unchangeable help code reuse them safely?

Answer: An update creates a new value without altering a shared original

An update prevents every possible mistake without tests — Unchangeable values cannot prevent mistakes in program logic. Tests still need to check that the code calculates and uses the intended dates.

An update makes every calculation run faster automatically — Unchangeable values do not guarantee faster calculations. Their benefit is predictable behavior when different parts of a program share a value.

An update creates a new value without altering a shared original ✓ — Creating a new value leaves the original unchanged for other code that shares it. Adding a day to a Temporal date therefore does not unexpectedly change the date another function is using.