User blog comment:NathanGMan/Phineas and Ferb Live In VIRGINIA?/@comment-68.35.198.237-20120401021605/@comment-3212462-20120401200801

Meaning, cartoon characters seem to inconsistantly exsist in the year they are needed to exsist in during the specific episode, particularly when the episode aired. They may be considered living in 2012 now, but next year the viewers would consider them living in 2013, the current time. As the characters don't age (it's only one summer the show takes place in), it can only be assumed that they are current, without labeling them with a year. So, their true birthdate can never be clear.