The 30 days begins when you receive the letter. The courts assume that you receive the letter within 5 business days of mailing, so you are good. There is case law on this, I just don't remember the case. Even if you assume that the letter was received on 2/18, 30 days means that you have until the 20th to send your dispute.

Dodging the 30 days period specified in the FDCPA is a common tactic.