they are celebrating David's 21st birthday in 1973, but David was born in 1950, so he would have been 23, not 21
After David learns of his father's death, which occurred in 1976, the next scene shows David in a bar having a drink. The sub-title reads 'One Year Later' as David watches Oliver North testifying at the Iran/Contra hearings on TV. North's testimony at the Iran/Contra hearings didn't take place until 1987.
When David is giving his famous Rolling Stone interview, which took place while the Partridge Family was still on the air, the song 'Hot Blooded' by Foreigner is on the radio. The last episode of the Partridge Family aired in August of 1974 but 'Hot Blooded' didn't come out till 1978.