A time difference is a result for a date, not an eternal property of a pair of cities. Seasonal rules explain why familiar gaps sometimes shift.
Zones change independently
Each jurisdiction decides whether to move clocks, on which dates, and at what local time. Neighboring countries may follow different policies, and countries in opposite hemispheres have opposite seasons. When only one of two zones changes, their difference changes by the same amount. When both change on different Sundays, a temporary gap appears between their transition dates.
Some local times are invalid or ambiguous
When clocks move forward, a range of wall-clock times is skipped. When clocks move back, another range occurs twice with two possible offsets. A converter should not guess which instant a user meant. It should identify the gap or overlap and ask for another input or, in a more advanced workflow, let the user choose an explicit offset.
Future answers can change
The IANA database records announced rules, but governments can revise future transitions. A conversion made months ahead is the best answer under the currently installed data, not a guarantee. Save the named zones with an event, update software and services, and recheck important plans close to the date rather than saving only a numeric UTC offset.