What times are used in minTemperature and maxTemperature in raw gridpoint data?

[mbg206]

I notice that the validTime property in the minTemperature and maxTemperature values show a time period for about every 12hrs, but what exactly decides the start times for these periods? (Time zone, actual forecasted temperature, does DST affect, etc.)

[sullynole]

The raw gridpoint data values are grouped into periods with the same value. Max and min temperature fields are a bit unique as they are usually forecast for 12h periods (max daytime hours, min nighttime hours) by WFO forecasters.