All of the World API data is timestamped to Coordinated Universal Time, or UTC.
If you are seeing solar irradiance or pv power generation occur at strange times, including overnight power generation, it is likely due to the fact that you have not converted the timestamps to your local timezone.
World API data is period averaged at 30 minute resolution. Each timestamp indicates the END of the averaging period and is recorded in ISO8601 date-time format. From our API documentation, the parameters in World API data files are:
If you need support in converting UTC timestamps to your local timezone, please checkout this help article.