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 WebForms there is a time stamp in the top right corner that shows when the form was submitted:
The following screenshots are for if you are on the updated layout for GovOS Studio. You will know if you are if you see a drop down for Account next to the Create button in the top right. If instead you see your username, then you are not on GovOS Studio 2.0.
Both of these can be changed by the author of the form by going to the Forms Manager (page 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 WebForm.