A method and system to help control when to monitor for presence of replaceable advertisements in a media stream. An example method includes determining a time offset based at least on a difference between (i) a time of occurrence of a content event in a media stream as indicated by data carried in-band with the media stream and (ii) a time of occurrence of the content event in the media stream as indicated by schedule data that is not carried in-band with the media stream. Further, the method includes applying the determined time offset as a basis to adjust a scheduled time of an advertisement in the media stream, and using the adjusted scheduled time of the advertisement in the media stream as a basis to control when to monitor for presence of a replacement advertisement in the media stream.