Very odd behaviour in 4.2.2 time and date

[BBNZowner]

My forum is acting in a way that I haven't seen on any other vb forums I've worked on before. I would post in the support forums however my account doesn't seem to want to verify as licenced (and submitted a ticket + response however no luck) and it's an urgent-ish matter as we are in out most important (opening) time, so any help is appreciated.

It seems that users are randomly being struck with date and time issues which means they both view and post in "the future". Generally 7 hours ahead of real time (the forum is set to GMT+13, and so it's a bit weird that you can be any more ahead of that!). It seems to be completely random as to who it affects and changes daily, guests generally see the correct time however.

All the individual users and the forum has the correct timezone set.
I've temporarily disabled plugins and hooks - no change.
Done the above and reverted to the default unmodified style - no change.
Set the server time via php5.ini and config to the correct timezone (shared host so no access to the actual php file) and this is being reflected fine via the phpinfo on the admincp.
I've looked for discrepancies in code in places where we'd assume the problem lies and personally can't find any (I'm no vB expert however).
We've even tried different browsers and OS out of desperation to no avail.

Any suggestions? I'm thinking just reupload all files from scratch and get it set up again. However if I import the tables/data is it possible the issue lies within that and the issue will repeat?