Detailed information about the exam start time is provided by the university or the testing organizers, in the instructions sent out.
There are 2 options:
1. If it was required to pre-register for a slot for the exam and you received a notification by mail from Examus with the text "Sign up for a control event".
You can enter the test no earlier than 15 minutes before the scheduled exam time. So, if you are signed up for the slot at 10:00, at 09:45 a button with the name of the available test and the inscription "Start" will appear in Examus.
The button will be available only after logging into Examus.

30 minutes after the start of the slot, the button will no longer be available and the entrance to the slot will no longer be possible.
If you are signed up for a slot at 10:00, then at 10:30 you will no longer be able to enter the test. Re-register for another slot by yourself or contact your university or test organizers with a request to postpone the exam.
Attention: Moscow time (UTC +3) is always used in the Examus interface.

2. If it was not required to pre-register for a slot.
In this case, you can start the exam exactly at the time announced by the university or the testing organizer. So, if you were informed that the exam would be from 10:00, then you won't be able to start before 10:00. Starting at 10:00, follow the link for the exam according to instructions from your university or testing organizer.

 

I can't see the button with the test name and can't start the exam

 

See also: Does the procedure for checking my computer and sending my passport photo use my examination time?