The act of baptism does not forgive sin (or save).  It is God who forgives sin and it is his prerogative to do it at any time he wishes. 

Before his death, Jesus forgave the sin of a sinful woman at the time she wetted his feet with her tears, wiped them with her hair, kissed them and anointed them with ointment.[1]  Jesus forgave the sin of a paralytic at the time his companions carried him to Jesus.[2]  He forgave the thief on the cross at the time he defended Jesus (“Do you not fear God, since you are under the same sentence of condemnation?   And we indeed justly, for we are receiving the due reward of our deeds; but this man has done nothing wrong.”), and openly declared that Jesus would rise again and was king (“Jesus, remember me when you come into your kingdom.”).[3]

After his death, Jesus chooses to forgive sin at the time of baptism.  40 days after the thief had died, Jesus commanded baptism (in Matthew 28.19 and Mark 16.16).  Obviously, baptism cannot be necessary for the thief (or for anyone before the command was issued).

[1] Lk 7.36-50.

[2] Mt 9.1-6; Mk 2.5; Lk 5.20.

[3] Lk 23:39-43.