Ad Manager only records conversions for users who have previously seen or clicked on an Ad Manager ad within a period of time that you specify, called a lookback window. There are two lookback windows, one for clicks and one for impressions. The lookback windows are set for each activity group. They can be from 1 to 30 days.
When a user loads a webpage containing activity tags, Ad Manager accesses the user's DoubleClick cookie. Ad Manager checks to see whether the user has seen or clicked on an ad within the lookback windows. To be counted, the ad needs to belong to one of the companies associated with the activity.
Effects of changing placement lookback windows
Example: Expanding lookback windows
You have an activity group with an impression lookback window of 5 days. Six days ago, a user saw an ad from one of the companies associated with an activity in that group. Today that user visited an advertiser's webpage with an activity tag. The activity tag sends a call to the Ad Manager servers, but it's not a conversion because it's outside the lookback window.
Subsequently, you change the impression lookback window to 10 days. This change has no effect on the website visit that already occurred. The next day, however, the user returns to the same webpage, triggering another activity tag call. Seven days have now passed since the user saw the ad. Since the impression lookback window is now 10 days, a conversion is counted.
Example: Shortening lookback windows
You have an activity group with a lookback window of 10 days for impressions. Four days ago, a user saw an ad from an advertiser associated with an activity in the group. Today that user visited a webpage with an activity tag, and a conversion was counted.
Subsequently, you change the impression lookback window to 3 days. This change has no effect on the conversion that was already counted. However, when the user returns to the same webpage the next day, no conversion is counted because the lookback window is now shorter.