What is the valid time of 12 HR Probability of Precipitation (PoP) charts?

The valid time shown on the 12 HR Probability of Precipitation (PoP) charts is the ending time of the 12-hour valid period. A chart valid time that says it is valid at 12Z on December 31 (Valid:12Z THU DEC 31 2015) means the forecast is valid from 00:00Z December 31 through 12:00Z December 31.

Image 1.  This PoP chart shows a valid time from 00:00Z December 31 through 12:00Z December 31.

In contrast, the long range Prog Charts are NOT valid over a period of time. Instead, they are valid at the time printed on the chart.