Mongodb 如何在不同时区按年月日汇总

我有一个MongoDB，它将日期对象存储在UTC中.我想在不同的时区(CET)按年、月、日进行聚合.

这样做对UTC来说很好:

    BasicDBObject group_id = new BasicDBObject("_id", new BasicDBObject("year", new BasicDBObject("$year", "$tDate")).
                append("month", new BasicDBObject("$month", "$tDate")).
                append("day", new BasicDBObject("$dayOfMonth", "$tDate")).
                append("customer", "$customer"));

    BasicDBObject groupFields = group_id.
            append("eventCnt", new BasicDBObject("$sum", "$eventCnt")); 

    BasicDBObject group = new BasicDBObject("$group", groupFields);

或者，如果使用命令行(未测试，我只测试了java版本):

{
    $group: {
        _id: {
            "year": {
                "$year", "$tDate"
            },
            "month": {
                "$month", "$tDate"
            },
            "day": {
                "$dayOfMonth", "$tDate"
            },
            "customer": "$customer"
        },
        "eventCount": {
            "$sum": "$eventCount"
        }
    }
}

如何在聚合框架内将这些日期转换为CET？

例如，"2013-09-16 23:45:00 UTC"是"2013-09-17 00:45:00 CET"，这是不同的一天.