Why doesn't she have to go to rehab immediately? A lot can happen in 45 days!
From what I understand, the judge did not give her a date but she has to show proof that she is enrolled in one by May 2 (her next court date).
http://news.yahoo.com/lohan-rehab-part-plea-deal-232843382.html
So I guess she has to be in rehab no later than May 2
and she'll be allowed (most likely) to do her rehab in New York and community service there too.
(since she lives in the NYC area with her mother)