The long range Prog Charts are valid at the time printed on the chart. A chart valid time that says it is valid at 00Z on December 31st (Valid 00Z Thursday December 31, 2015) is valid at 00:00Z on December 31st.
Image 1. This long range Prog Chart shows a valid time of 00:00Z December 31st.
In contrast, the valid time shown on the 12 HR Probability of Precipitation (PoP) charts is the ending time of the 12-hour valid period. A PoP 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.