I think you may have missed the idea: usually dates (not datetimes) are the legal fiction "date" NOT "an instant." I.e. timezones are irrelevant.
Birth dates, contract dates, sale dates, billing dates, insurance coverage dates, etc.
---
EDIT: UTC still plays a role -- as there is still the choice of calendar (though you'd be forgiven for assuming Gregorian) -- but it's an odd statement to decipher.
Ah! You're right, I totally missed that the commenter was griping about UTC dates as opposed to datetimes. I agree, a "UTC date" is not a clean concept. We can talk about Julian or Gregorian dates, but those are independent of time scales like UTC or TAI or UT1.
> Nobody is going to adjust your birthdate when you move abroad.
But that’s just a convention, right? The day you are born is still dependent on the time zone. If you are born in the US at 11pm EST then someone born at that same moment in the UK has a different birthday.
Dates have boundaries. These boundaries are dependent on time zone. We can talk about dates irrespective of time zone but day periods cannot be understood without reference to time zone.
You are mixing two things up into one conversation. You are adding time into the conversation, in which case yes you need timezones, but if you don't add time into the conversation and just have dates, then you don't have timezones.
UTC isn’t a time zone, its a specification for how many seconds are in a day. In UTC, there are 84000 seconds on most days, but the IERS may announce a “leap second” which makes some particular day either 84001 seconds or 83999 seconds (historically always the former).
Birth dates, contract dates, sale dates, billing dates, insurance coverage dates, etc.
---
EDIT: UTC still plays a role -- as there is still the choice of calendar (though you'd be forgiven for assuming Gregorian) -- but it's an odd statement to decipher.