Yeah, thats right. Me and my friend actually got into a long conversation about Einstein, relativity, and time travel because of this book he stole from a library by accident. Apparently, if you are traveling at the speed of light, time moves DRASTICLY SLOWER. A good example is from the 'Ender's Game' series by Orson Scott Card. When they travel at the speed of light from one planet to another what takes teh travelers 20 minutes actually takes the people outside the speed of light 20 years. (not exact proportions but I'm just illustrating the idea here.)
So thats why she can Merge with Lavos in 1999 AD when being sent from 12,000BC because time is moving so slow for lavos that the difference from 12,000 BC to 1999 AD would be insignificant (if thats how the DBT works anyway. Buts its all just a theory and cannot be proved, yet hopefully it will be someday!