It goes by the time zone you are filing from. Please don't go down to the last possible second----something is almost bound to go wrong if you do that and then you will not be able to e-file. If you miss the deadline your return has to be printed, signed and mailed.
If you e-file and it is rejected you get a few days to fix it and re-file, but you cannot e-file for the "first" time after the deadline.
If you are getting a refund or do not owe there is no penalty for missing the deadline; you have three years to file. If you owe tax due you will be subject to late penalties and interest if you miss the deadline.
**Disclaimer: Every effort has been made to offer the most correct information possible. The poster disclaims any legal responsibility for the accuracy of the information that is contained in this post.**