Each of your level 1 site is assigned a timezone, corresponding to the country that you selected when filling up the site creation form. This timezone affects the way any date-time analysis is shown in the interface (as well as real-time data), so be sure to keep that in mind when analysing specific time periods.
Should you wish to change the timezone, be aware that you will have to switch to another data collection log. In this case, you will have to update your tag.