Javascript SDK with convertDateTimesToDates=true does not convert localDateTime

This is by design. The time zone offset for an activity file is calculated from the delta of the timestamp and local_timestamp in the activity message. Otherwise “local time” would be based on the time zone of the system decoding the file and not the time zone of the activity. Historically the activity message is the last message in the file, so you don’t know what the time zone offset is until all messages are read from the file. There is a section in this recipe on “Converting Timestamps to Local Time” that covers some of this.

https://developer.garmin.com/fit/cookbook/decoding-activity-files/

If there is a need to convert local time stamps to JavaScript Date objects, that can be done using a message listener. That would look something like this.

import { Decoder, Stream, Profile, Utils } from "@garmin-fit/sdk";

const onMesg = (mesgNum, mesg) => {

 if ("localTimeStamp" in mesg) {

   mesg.localTimestamp = Utils.convertDateTimeToDate(mesg.localTimestamp);

 }

  // Or...

  if (mesgNum === Profile.MesgNum.ACTIVITY) {

     tzOffset = (mesg.localTimestamp ?? mesg.timestamp) - mesg.timestamp;

 }

}

const { messages, errors } = decoder.read({

 mesgListener: onMesg,

});

In my own projects that use the JavaScript SDK, I always set convertDateTimesToDates=false. The reason being is that there are a lot of timestamps in a file, but only a few that are ever shown to the user in local time so I just convert the ones that are shown to the user. On a chart there could be10k data points but only 10 x-axis labels. So I will convert the FIT timestamps to JavaScript Date objects in the axis label callbacks. This saves some processing time during decoding, saves memory w/o all of the Date objects, and makes other calcs faster and easier since the timestamps are numbers and not Dates.