An online advertising system receives ads and ad exposure goals, such as a desired number of impressions or amount of presentation time, from advertisers. The online advertising system may also receive a time-based advertising purchase. A request for an ad is received for a client, and an amount of exposure is determined for each received ad across a plurality of ad mediums. The ad mediums may include both controlled ad mediums and external ad mediums. The advertising system selects an ad for presentation among the received ads based on the received ad exposure goals associated with the ads and the determined amount of exposure for each ad. The advertising system may select an ad by determining bids for each ad and conducting an auction among the ads. In such an embodiment, the advertising system may determine the bids for ads that have met their ad exposure goals to be zero.