Hello,
Sorry to hear you're having issues.
The extension does not use your IP address.
Browsers come packed with an API for Internationalization, that uses your system information.
When processing times, the extension first normalises them to UTC, before applying your timezone offset, which is determined using the Date getTimezoneOffset function. They are then formatted using Intl.DateTimeFormat.
I would suggest checking that your system time is correctly set. (Perhaps DST is incorrectly set)
Also, just in case you have been testing times specifically using EST, most places in the Eastern timezone are currently using EDT, not EST.
The extension understands the use of ET as shorthand for the currently used version.
(I do want to be able to warn users about these mix ups in a future update)

I will consider adding an option to set your desired local timezone.

I hope this helps you, if not please do get in touch.