Hello there,
Welcome to our forum and thank you for contacting us with your question.
Although you want be able to use both of these properties together, there is a simple workaround for that.
Instead of using both of them, you can use
time.getTimezoneOffset callback function, in which you get to choose a timezone and then you can add to it a custom offset.
DEMO: https://jsfiddle.net/BlackLabel/nc15Lryw/
API Reference: https://api.highcharts.com/highstock/ti ... zoneOffset
Let me know if there is anything else I can help you with,
Best regards!