I agree with pp. We've done early BTs *a lot* in this house; during the 2-1 and now during the 1-0. But, lately I don't go any earlier than 6:45 pm (maybe 6:30 if she desperately needs it) and she has been coping ok. It's just not practical to get her down any earlier and 12 hrs is about the longest night we get.
I would try for an earlier nap on non-preschool days, maybe by 30 min to an hour. I think if you go too far off at this age, it doesn't jibe with their internal clock. I also agree with a later, but shorter nap as well. I've found 45 min - 1 hr between 2 and 3 pm doesn't ruin bedtime for my DD as she's just tired from no nap days and the long morning, and it does help her to have a more restful night. Sometimes those late ones happen in the car, though, because she's too tired to get herself to sleep in her bed.