I realized that on the activities section I put the duration of my work experience as 09/15 but it should have been 09/14. Is that a big deal? like they would reject me for that? I emailed them, but are they going to fix it? btw I applied to UCLA, UCB, UCI, UCSD
No, They will not reject you over this minor typo. I can’t imagine it would have any impact at all. There is probably not one application out there that is devoid of any and all mistakes.
But would they dislike it and perhaps affects my admission?
No it should not impact your chances.