This also affects datestamps for sending email invitations/reminders.

@mazi : basically : can be a By Survey settings , A lot of config part can be updated using Yii::app()->setConfig('timeadjust',$newvalue);

The question here is how you get the good time difference for all user ? There are a existing system ? By IP ?

@DenisChenu, for a first start I would simply let the admin define the time difference for a survey. If nothing is set, we take the default setting from th global settings but if the admin know that the survey will be done e.g. in Australie, he can simply enter a different value at survey level.

About token : https://manual.limesurvey.org/BeforeTokenEmail , and inside survey : https://manual.limesurvey.org/BeforeSurveyPage : i thinnk it work.
You can easily try if it work without any config , set to 10 on all surveys ;)