Time in calculus of satellite position from ephemeris data

Hi,
I'm trying to determine the position of a satellite from ephemeris data but i'm facing several problems, if anyone could help :

1. From what i can find on the internet, the first thing to do is to compute time tk as :
tk=t−toe
toe is time of emission given in ephemerides, i can't understand what does exactly t refer to ? is it time of transmission (also given in ephemerides) ?
cf Navipedia :

Code:

gssc.esa.int/navipedia/index.php/GPS_and_Galileo_Satellite_Coordinates_Computation

2. Second problem comes when i want to calculate satellite clock offset

Code:

gssc.esa.int/navipedia/index.php/Clock_Modelling

as:
δt~sat=a0+a1(t−t0)+a2(t−t0)^2

first, i think t0 is the date given at the beginning of each "block" of navigation data (but i'm not sure about this)
and once again, what is t ??

Thank you