So if you look at these two variables at the source code carefully:
var g_originalEstimate = 70737 + ( Math.round( new Date().getTime() / 1000 ) );
var g_updatedEstimate = 50921 + ( Math.round( new Date().getTime() / 1000 ) );
var g_originalEstimate = Original countdown.
var g_updatedEstimate = New countdown.
Someone is controlling this variable as you notice there is an arbitrary number listed in there as solid numbers are listed in seconds (3600 seconds in an hour). Do the math if you don't believe me.
The numbers 70737 and 50921 should be changing as those were mine at the time of writing this message.
tl;dr Valve will release Portal 2 within 24 hours.