when to file for US citiznship

[mehta_agm]

hi. My parents migrated on feb 3' 2003 to US on green card. In between they had travelled once for 4 months to india. Wanted to know when can they technically file for US citizenship. Also on my mother's green card there is a small biographical error on Birthdate ...the actual date of birth is 8th july.. But on green card it shows 7th Jul. Can i get this error corrected before applying for citizenship or after she becomes US citizn.
pls advise.
anand