Well, the speed of light is never going to change. That being the case, one method could be to use light years as a means to calculate time.
'x light years from now' could be the way to specify a relative date.
You could also include how much time a second is (via a gap in the audio) and specify that 60 seconds make a minute and 60 minutes make an hour and so on. Using that you can explain our concept of the passing of time and use the same to explain what you need….
if they can pick up the signal, they should be able to find where it came from. so you can use the speed of light and the distance between us and them to give them an idea of how long ago the message was sent to them.
I seem to have made a mistake: light year can only be used to measure distance and not time. so my first option can be discarded.
once i went through the question, i realised one fact: our concept of the big bang and when it happened might not necessary be what they believe in. so using the big bang as the reference might not be the way to go.
We could measure the distance travelled by the satellite and use the speed of light as reference to convey the time taken by the satellite to get to the aliens with reference to the launch of the satellite.
that is to say, as long as the satellite had good enough sensors, the satellite could find the distance between local astro bodies and specify that the (satellite) was launched x units of the time taken to go from one astro body to the other.