The default timezone for Social Report is CST - US Central Timezone.
Each project however can have its own custom timezone. When you are creating a project you are offered to choose the timezone that works best for you. You can also change the project timezone at any time by going to Settings > General.
Setting the project level timezone offers a default timezone selection to all project users.
Each user however may decide to see project data in their own timezone. The user can change their project level timezone by clicking on the date range selector and choosing from a list of timezones there.