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.
This PoP chart shows a valid time of 00:00Z—12:00Z on December 31.
In contrast, Prog Charts are NOT valid over a period of time. Instead, they are valid at the time printed on the chart.