The timezone that the user will experience on that date.

>Either way, to store an appointment for a certain user, you need the local timezone of that user and the local timestamp.

Correct. I did not disagree with this.

>Local TZ -> UTC is a formula that is not constant. You cannot store the output of this formula and expect to reverse and get back the original input precisely because of the changing formula.

Also correct. But as I said, your original comment implied this is a concern for all DST events which is not true. It is only a concern when the time was converted using some DST assumption and then that assumption is invalidated (eg the software decided to convert "4pm Tuesday local" to UTC assuming the timezone on Tuesday will be UTC+7, but the DST change happens to come early this year and in fact it's UTC+8 on Tuesday).

Regardless, I agree that taking this risk is not correct. Such a time should not be converted to UTC to begin with. Better to store the original local time.