Is there any paper submission deadline timezone convention?
Primarily, you should really try to find out what time zone a conference uses rather than assume a specific one. As ff524 says, even if it is not on the web site, the submission site will often tell you.

That being said, at least in my discipline, there are two conventions that are common:


Local time at the conference location
AOE (anywhere on earth), more formally known as Howland Island time.


The latter is probably the more common one.

(Howland Island time is called "Anywhere on Earth" because it is the last time zone. That is, if you say that a deadline is friday, 24:00 AOE, the real deadline will nowhere be earlier than friday, 24:00, reducing the possibility of feel-bad moments when paper writers forgot about time zones and assumed their own time zone.)