There might be a setting for automatic "daylight saving time". I live in Toronto, Canada, which is also Eastern Time Zone. We are now in Eastern Daylight Saving Time (GMT - 4) instead of Eastern Standard Time (GMT -
5). That's why your GPS keeps putting your clock back to Eastern Standard Time.You should remember you have done this, otherwise when you plug it into a computer to update the map it will only charge your GPS.
Go to the page on Amazon.com for your GPS. Quite often Amazon.com will put a link of the User Manual on the left hand side of the screen somewhere down the page.
Amazon would do it on Amazon.com, but not Amazon.ca. Sometime I have to switch over to Amazon.com to download the User Manual.
If you cannot find it on Amazon.com, then try to Google with keywords like "GPS [insert name and model] user manual PDF". I have lots of success finding the User Manual I want.
Besides "automatic Daylight Saving Time" setting, You might be able to find "automatic time zone setting" too.