How to pass datetime from client(browser) to server

I want to pass datetime from client to server. I prefer to pass in ISO format using .toISOString() method. The challenge is while passing datetime(Javascript) to server, if I used C# DateTime then it tries to convert datetime in local datetime. So to get it as in UTC I am using DateTimeOffset. But by this I am not able to get exactly what was client’s offset.

In format I should pass datetime to server so I can retain offset of client or should I pass it separately?