The Albertan could have set the time to 12:00 in Europe/London, but probably few people do this unless they're expecting to travel to the other place.
1. the London person owns it, schedules in local (London) time 12:00
2. the London person owns it, schedules mental math explicit Alberta time 05:00
3. the Alberta person owns it, schedules in local (Alberta) mental math time 05:00
4. the Alberta person owns it, schedules in explicit London time 12:00
You cannot get all of these working by software only no matter what you do, because there "mental math" time cannot take into account the implicit expectations, 1 and 4 will be correct as stated (the meeting is at 12 London time) but 2 and 3 what do you do? How would you differentiate between "mental math what I thought Alberta time was" vs "I really meant it at this Alberta time"
Consider a Doctor's appointment for Dec 20, 2026 2pm in Calgary. Regardless of whether its booked a year ahead (ie before the new time zone rules were decided) or a week ahead (after the rules were decided, and consequently a different UTC time), the expectation is that it will be at at 2pm local time.