Whenever an IRS Form is E-Filed online, the form is marked with an Electronic Postmark bearing the date & time
the return was filed according to the transmitter's time zone.
Depending on your location, you may need to adjust the Electronic Postmark to the time zone where you, the taxpayer, reside in order to determine the postmark's actual time.
For example, when a Tax Extension is E-Filed through ExpressExtension.com the Electronic Postmark is generated in Eastern Standard Time. If you are filing from another time zone, then the actual postmark would be the time in which the return was E-Filed according to your Time Zone.
If you are E-Filing from another Time Zone, your return is still "On Time" if it is filed by midnight, your local time, on the deadline date.