During the upgrade to OpenX 2.6 you will learn that with this version statistics are saved now as UTC, opposed to previous stats that were saved in the database with the specific timezone configured. During the upgrade process you can choose to disable the warning for this change in your installation. Still if you have chosen to leave this on, and later on annoyed by this warning that will appear on all your statistics screens, you will want to disable it, you will find out that there is no admin setting or config to disable it.
If you want to disable the Utc timezone warning you will have to run the following mysql statement (this will just rename the variable, and not remove it completely):
on the openx database.
Here is a real life example with the database called openads and the tables prepended with phpads_:
1 2 3
Doing this will just disable the warning, and has nothing to do with your old saved statistics. If you want to migrate them to UTC from your pervious timezone configs, then just follow the openx docs: http://www.openx.org/en/docs/2.6/adminguide/Upgrade+Time+Zones