What is the exact meaning of a deadline for conference paper submission?
There are three typical scenarios that you will encounter with most computer-science-style conferences (including most IEEE conferences):


Submissions shut down at the exact time specified, and nothing late will be accepted.  This is most common in large top-notch conferences with extremely fierce competition.  If this is the case, a time-zone will almost always be specified as well.
Submissions shut down an arbitrary but relatively short time after the deadline, i.e., when the organizers get around to switching it off.  There is still likely to be little flexibility, but the conference is less high-stakes, and so they aren't worrying about precision.
Submissions will actually be due ~1 week later, after a universal extension is announced.  This is most common in smaller conferences that are more concerned with making sure they get enough submissions.  It can also be counter-productive, since a tradition of extensions means nobody takes the first deadline seriously.


If you see a time zone, you're probably in case #1; if multiple past editions show an extension, it's probably case #3; otherwise, it's likely case #2.