Whenever you are accessing your submissions via the Submissions Manager you will notice that there is always a Submission Created column on the left hand side:
Additionally, on Web Forms there is a time stamp in the top right corner that shows when the form was submitted:
Both of these can be changed by the author of the form by going to the main lobby when you first log-in and clicking on your username in the top right and then clicking on Profile:
Once you are in your profile there will be a section where you can change your Time Zone for all submissions on the form you are the author of:
NOTE: As mentioned above, only the Time Zone selected in the profile of the Author of the form will reflect in the Submissions Manager and on the completed PDF of a Web Form.