If I create a Due Date Automation with the trigger
2 days before a card in list "List" is due at 12:00 pm
It will execute at noon.
But if I create a Due Date Automation without a time specified
2 days before a card in list "List" is due
It does not execute 48 hours before the card's due date & time.
So I am guessing that perhaps the trigger is not set up to execute at the exact specified interval before the card's due date but perhaps it executes at a default time, such as midnight local time, or midnight GMT/UTC, or some other time.
In any case this is not covered in the documentation, nor could I find an answer in the existing community responses.