approved excuse letter from dept chair

hello i'm hoping someone knows how this process goes? absent kasi ako sa subject ko na may quiz but i was supposed to attend the class talaga pero dahil umuulan ++ MAY STRIKE nalate ako ng dating and hindi na ako nakapag quiz. nirerequire ng prof namin magpasa ng excuse letter approved by the department chair... ano po process nito, how long will it take and will I be able to achieve this on the same day? TYA