How do time zones work?

Can someone tell me if this is correct? I know the GPS records all times in GMT and the program has to convert the times to the current computer's time zone, but I got some unexpected behavior.

When I view the points in a track, the time it is displaying is for the time zone of my computer, but adjusted for daylight or standard time depending on when the track was made, regardless of whether it is daylight or standard time when I view it. So if I record a track in daylight time and view it in the summer when it says 6pm, then if I view that track in the winter it will still say 6pm even though my computer is now set to standard time.

So the program must be making the DST adjustment depending on date the track was recorded, regardless of when I've viewing it. Maybe that makes sense, but not if it's doing this behind my back. It's especially confusing when I created the track in a different season in a different time zone, as it's using my current computer's time zone but still doing the DST adjustment, as I get a time that makes no sense. What if my computer is in a location doesn't observe DST?

It would be a lot more straightforward if it would just let me set GMT offset to use when displaying times.

0 Former Member over 12 years ago

All gpx and tcx files have GMT time recorded in them. I use WinXP so I do not know what Mac uses but I use WordPad or NotePad++ to open gpx and tcx files to read and edit them.

You said "When I view the points in a track, the time it is displaying is for the time zone of my computer, but adjusted for daylight or standard time depending on when the track was made, regardless of whether it is daylight or standard time when I view it. So if I record a track in daylight time and view it in the summer when it says 6pm, then if I view that track in the winter it will still say 6pm even though my computer is now set to standard time."

You are correct that the time is being corrected to the time zone of your computer but at the DATE the trackpoints were recorded. The date and time you view the track does not change the time the track was recorded. A track recorded at 6pm in July will stay at 6pm no matter when you view it.

0 mgasser over 12 years ago

0 Former Member over 12 years ago

The correction made by the application makes sense on first blush, but borders on "mysterious" when the track was recorded in a different time zone from that of my computer, and months ago when DST was different. Does BaseCamp actually keep track of the dates of DST for all years in the past, in all jurisdictions?

No it does not keep track of DST dates.
Every tcx and fit file is recorded in GMT along with a timezone number based on the timezone that the gps was in at the time of recording. For me in Calif, USA, PST, my number is (-8). If I open my tcx file with a text editor, I use WordPad, I will see at the top encoding="UTF-8".

You are not exactly right that it will say 6pm no matter when I view it. That's true providing my computer is in the same time zone as the track point. If I view the track on my laptop and change my laptop's time zone as I travel, then the track point's time will change.

Yes the computer has changed the time for the points in the track but the time is still correct for that computer.
Lets say I have a track "A" recorded in CA at 8am. The tcx file will have encoding="UTF-8" in it.
If a person in Virginia EST (-5) opens my track "A" their computer will show the time as 11am which is correct for that computer.
A person standing in VA at 11am and looking at my gps in CA would see the time as 8am.

It sure would be nice if the application gave me the option of specifying the time zone, because what I really would like to see for time stamps is the local time at that track point.
If the track was recorded in DST and I'm still in DST, then I can accomplish this by changing the time zone of my computer to that of the track location.
But if the track was in DST and it's now standard time, I have to play mental games with my computer's time zone to see track points in the correct time.

Yes, changing the timezone of your computer to the timezone of the track as shown in the tcx file, will guarantee that your computer is showing you the correct local time for that track as long as the DST date changes are the same for both time zones. A web search for DST dates for each time zone will help you adjust the local time correctly if you want to get it exact.

Best would be if it automatically used the time zone based on the location of each track point, as my GPS does when it displays local time.
But even if it does such a thing automatically, it needs to let me find out what time zone it's using for any given track point if I need to be sure, or allow me to see GMT which is a constant.

Again, you can see the GMT time for any track by opening it with a text editor.
For an automatic solution you need someone a lot smarter than me to write you a program that will massage all these times and zones and dates.

The easiest, non-automatic solution, is to just change the encoding="UTF-8" number in the tcx file to match the timezone number for your computer.
You will still have the DST problem if the DST dates are different or if the state does not do DST like Arizona.

0 JaVaWa over 12 years ago

Encoding="UTF-8" has nothing to do with time zones! If you change that number, BaseCamp won't be able to read the file anymore.
You can read here what it means: http://en.wikipedia.org/wiki/UTF-8
The GPS doesn't put time zone information in the file.

0 Former Member over 12 years ago

Encoding="UTF-8" has nothing to do with time zones! If you change that number, BaseCamp won't be able to read the file anymore.
You can read here what it means: http://en.wikipedia.org/wiki/UTF-8
The GPS doesn't put time zone information in the file.

Yes, you are correct. I should have tested a tcx file of mine before I made that assumption.
That "-8" seemed so obvious to me because I am in timezone "-8".

Thanks for the info.