Whenever an IRS form is E-Filed online, the form is marked with an Electronic Postmarkbearing the date & time the return was filed according to the transmitter's time zone.
Since Different E-Filing Organizations have different time zones, you may need to adjust the Postmark to your Time Zone.
For Example, when a Tax Extension is E-Filed through http://www.ExpressExtension.com the Electronic Postmark is generated in Eastern Time. If the filing party was filing from another time zone, then the actual postmark would be the time in which the return was E-Filed according to the filing party's 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.