24 hour clock? Is it possible?

[routeconverter]

The time in the GPX file is already in 24 hour clock format, isn't it?

Time in most parts of the world and in most software is in 24 hour clock format. SCNR ;-)

If so, why does it need to be converted to 'locale' format?

Actually it's the output that does the conversion. Internally time is stored in milliseconds since 1/1/70 and the timezone. To format it into a string, a date formatter has to be used. And by default this date formatter uses the locale to present English users their AM/PM stuff, Germans their day then month then year preference etc...