The difference, in minutes, between UTC and local time. This means that the offset is positive if the local timezone is behind UTC and negative if it is ahead. Changing the timezone settings on the machine might cause the timezoneOffset to change within the session. Daylight Saving Time prevents this value from being a constant even for a specific locale.

{
    "messageVersion": "1.1.0.0",
    "serialNumber": 2,
    "sessions": [
        {
            "id": "ID13H56M10S772R0.955239302458247",
            "startTime": 1343076970772,
            "timezoneOffset": 420,
            "messages": [...],
            "clientEnvironment": {...}
        }
    ]
}