The Cassini probe is going to be taking a picture of the Earth today, July 19th, at 4:27pm CT. It's 80 light minutes away currently, and this time is calculated based on the time that it will actually take the picture. So at 4:27pm CT, the light generated from our planet at that time will be what ends up on the exposure.
I like when NASA gives us this kind of information, as it's just kind of fun to think about and feel slightly involved. If you ever see that picture in the future you can say "I'm on that spec, and I'm waving!"