I would like to use this service to adjust time returned by NTP for the timezone and DST. Is this possible with the timezone web service? It is not clear to me, for example, if rawOffset is adjusted for DST or not.
The timezone webservice primarly returns the timezoneId for a given lat/lng.
If you have the timezoneId then you can do the adjustment locally, as long as you have the newest timezone rules installed.
If you do not have the timezone rules installed or cannot make sure that they are updtoday then it might sense to use a third party service. The current dst adjusted time is in the time field, the offset fields refer to 1. of Jan and 1. of July. They don't contain the information at which point in time (month, day, hour) the change occurs.