It's more complex, but usually it better matches user intent. If I say the conference starts on 2027-04-05 13:09 in Berlin, and Germany decides to change their time zones or daylight saving time rules sometime in 2025, your stored offset or stored UTC time is now wrong. The conference will start whenever local clocks show 13:09, and you have no reliable way of knowing which time zone local clocks will be in in two years time.
Of course if we go down that path we should also add a format to indicate offsets. If I say I'll be back in 10 minutes, that interval shouldn't change no matter what happens to the time zone. To perfectly disambiguate it you would need to store it as current time (with timezone or UTC) plus the intended offset. This would also "solve" a lot of issues around leap seconds and the inability to predict them more than six months into the future.
Of course if we go down that path we should also add a format to indicate offsets. If I say I'll be back in 10 minutes, that interval shouldn't change no matter what happens to the time zone. To perfectly disambiguate it you would need to store it as current time (with timezone or UTC) plus the intended offset. This would also "solve" a lot of issues around leap seconds and the inability to predict them more than six months into the future.