Updated as needed. I will add to this page as more stories are added. I don't want to give away too much, otherwise you'll have no need to read the stories.
Real-time Universe
The here and now. For example, if you're reading a story and it's 2004 and Jen and Lauren are 13, you're in the real-time universe. I have decided that this time ends on June 2011. Nothing bad happens to the girls, the world doesn't end, just the time in this universe ends then, for the purpose of our Las Almejas adventures. The present is boring.
Floating-time Universe
The "other" universe. If you're reading a story, it's 1993 and Jen & Lauren are 19 and Fiona is 20, you know you're here. This universe can encapsulate anytime in the past, present or future, though for simplicity's sake, usually it takes place before the girls were born, or when they were very young. Future stories will most likely take place in a future time, though that still isn't set in stone.
The overlap
Holy SHIT, don't even joke about that!