The USA reverts to solar time on the first Sunday in November.

to be fair the USA reverted to 1930's time in February and doesn't look like changing any time soon :-)

I have just registered but had no option to add event to calendar.
On the notification did it give a time zone?
Serif is UK based and Canva is Australian....

Having signed up I got an email saying "Want a glimpse before it begins?
Keep an eye on our social channels. We’ll be sharing glimpses of what’s about to unfold. You’ll find the links in the footer"

The socials being Farcebook, Xiter and Insta,but not on the Affinity forum it seems nor their replacement discord-chatter.
Are the going back down market again?

There is also nothing on the Canva web site. So I would assume it will be a UK timed announcement?

I think … that people in America will not have put their clocks back by 30 October

You can use a website such as timeanddate.com or time.is to find the answer to that question.

I have registered some time ago. I seem to have missed that feature.

So that seems to indicate 6 pm UK time, except that we but the clocks back on the last Sunday in October, so it might be 7 pm here as I think, but am not congruently sure, that people in America will not have put their clocks back by 30 October.

When I was young, UK clocks used to go back in September and mainland Europe clocks went back at the end of October.

It caused problems with imetables for ferry services across the English channel.

I'm on the east coast USA where Eastern Standard Time is a three hour difference from Pacific Time. My Canva calendar entry indicates a start time of 1pm, which is a three hour difference from the stated 10am PT start time.

If you register, you'll receive a calendar entry with the correct time for your region.

The UK will put the clocks back on the Sunday before.

Is the announcement being made worldwide simultaneously?

I have seen mention of 10 am PT and mention of 10 am Pacific Daylight Time.

It seems American practice is to put the clocks back on the first Sunday in November.