Continuity mistake: When they visit Morgan Freeman at his office the yellow handkerchief in his jacket pocket appears and disappears frequently.
Continuity mistake: As Ted and John consult with a lawyer to take Ted's case, a silver box and an open book are either close to or further from a candy dish on the lawyer's desk as the shots change.
Continuity mistake: The money the guy puts on the counter when asking about the Trix disappears with no one touching it. (00:14:00)
Continuity mistake: Sam's shoes are light colored high heels while she is sitting on the stairs with John talking to Ted before he got mad and left, eventually ending up in Comic Con. They're black flats when she and John are looking for Ted all the way through to when John was hit by the Enterprise.
Continuity mistake: The hands of the adoption counselor Ted and Tammy Lynn are consulting with change positions from clasped to flat on her paperwork and back again as the shots cut.
Continuity mistake: When John falls over at the clinic, the wetness of his shirt changes drastically. (00:29:00)
Continuity mistake: Near the start in the bar, the blue label on John's beer keeps disappearing and reappearing instantly. (00:12:30)
Continuity mistake: When John and Ted are doing Thunder Buddies at the start, the amount of stage monitors on the stage keeps changing from 5 to 2 to 0 and back depending on the shot. (00:05:50)
Factual error: Ted is driving the car and goes airborne into a barn where the car comes to a sudden stop. Samantha, who is sleeping in the back seat without her seatbelt, doesn't shoot forwards from the rapid deceleration.
Continuity mistake: The way John is holding the red cup when he lifts Tom's covers off him changes instantly. (00:23:40)
Revealing mistake: When Ted and John argue in New York, Ted walks off across the road and disappears behind a passing car. Look closely - Ted disappears a fraction too early.
Answer: Films are basically never set exactly when they're released. Most likely they just picked a random date around the likely time of release, but scheduling changed. Alternatively they just picked an arbitrary date in the future for the newspaper, but a date in the recent past would have worked just as well.