Site Time Zone & Local Standard Time

I would like to know what site time zone has to be defined for a place in China. Legal time zone, is UTC+8 but the location for my CSP study is closer to UTC+6 or 7.
What influence has this value for production studies?
what is the reference to calculate solar angle?
We define the Longitude and latitude, but I don't know what time stamp is the correct for this place.
I have an error message depending on time zone with me own TMY.

Thank so much