Help! will the offer be withdrawn due to the wrong employment dates on CV

The Graduate
#1

i got a offer from a IB of 2012 in the campus recruiment in japan.
and one week after i recieved my offer i found i put the wrong dates in my employment history.
i have been worked from 2008.10.1 to 2009.7.30 before i went to the graduate school
but i wrongly wrote 2008.10 to 2009. 9…
and actually i wasnt working in 2009.8 preparing for the entance exam, and in 2009.9 i got the admission from the graduate school.
i think they will sure find out in the background check.
so will it be a big problem?
im really afraid of them taking this as a lie in CV thus withdraw the offer…
what should i do???