I have a GPX file that somehow has the date/times out of order. Probably created by a “merge.” I once tried a couple of programs that claimed to “merge” GPX files when really they just appended one to another.
Where can I find a program in a web app or for MacOS that will re-order the trackpoints by time in a GPX file?
A web search found two that apparently have been deleted or relocated and one that had confusing documentation (apparently uncorrected errors).
GPS Babel might be able to do that.
The page is written in German, but declared as English, which makes included translation widget useless.
I tried this manually edited URL to get that website in English:
RouteConverter - Home
And the software itself has includes 22 languages?
see RouteConverter - Translated in 22 languages
only if it’s stupid, because it should be very doable for a translation system to recognize the language used.
Well, I’ve tried translating and the tool was clearly trying to translate German text from English.
But the link provided by @stephan75 https://www.routeconverter.de/home/en works perfectly. This should be the default 
I’m using https://getadze.com/ for fiddling with GPX tracks, maybe it can also re-order trackpoints.
The method I ended up using is the “convert” at gpsvisualizer.com
Convert to plaintext, sort -u, move the header back to the top, then use the same converter to put it back in GPX format.