public ExpressionConverter setTimeOffset(int millis) {
        this.timeOffset = millis / 1000;
        return this;
    }
Sets an offset to use for all time-based grouping expressions.

@param millis The offset in milliseconds.
@return This, to allow chaining.
container-search/src/main/java/com/yahoo/search/grouping/vespa/ExpressionConverter.java