Is AddThisEvent's timezone (zonecode) arbitrary?
I have users stored in the database and need to save a timezone to associate with the user so that I can use it to create events with AddThisEvent and to display other localized times.
Is the mapping from a timezone offset to the AddThisEvent zonecode arbitrary- or is he using some standard that I couldn't find?
The documentation in Step 2 describes the list of available values for the _zonecode.
These appear to be Microsoft Windows time zones, as mentioned in the timezone tag wiki.
However, the actual integers assigned to each "Zonecode" appear to be proprietary to AddThisEvent. It's likely that they are using TimeZoneInfo.GetSystemTimeZones in .NET, and assigning each entry to an integer.
Hopefully, they realize that this list is not necessarily fixed. Microsoft can provide updates which add entries to Windows when new time zones need to be created. For example, two new time zones are being added soon for the upcoming October 2014 Russian time zone changes. These would be added in the middle of the list, so the list would no longer be sequential. If they are just using an index value of the array returned, they'll be surprised when the changes cause everything to shift forward.
(Note - I have no special knowledge of how AddThisEvent is actually planning to handle updates. I am just speculating.)
In general - integers are not good time zone identifiers.
So what should I do then?
Map each of the proprietary entries back to the Windows time zone they belong to. Then, if necessary, map them to the standard IANA TZ identifier used by PHP, Python, Java, etc. You can use the CLDR mappings to determine an appropriate TZ identifier to use for each Windows time zone.
It's a shame you'll have to jump through so many hoops, but in the end, you'll be able to figure out the appropriate integer to use with AddThisEvent for each user.