Are you writing an add-on? What kind of issue are you having with timezones?
To be clear about what I was talking about above, I was developing an add-on that needed to use today's date in a database query.
I initially did this using the PHP function "date", but that didn't respect the visitor's preferred timezone because of the way XenForo_Application sets the PHP timezone to 'UTC'.
Instead, anyone developing a XenForo add-on that needs to access the current date should use the 'date' function in XenForo_Locale.
This will use the visitor's preferred timezone to decide which day it is.