What is the sent date time in Campaign Monitor?

A campaign's sent date/time is typically when a Campaign Monitor campaign is sent. However, there is a feature called optimised sending. With optimised sending, although email send will be started at a particular date/time, people will receive the email within a 24-hour window. The sent date/time may reflect the time the campaign was sent to the last recipient in that window, not the time it was started. Therefore, if you are using our sync to Salesforce Campaigns, the sent activity may be recorded after the clicks and opens. 

Related FAQ: Why are Salesforce Campaign response rates incorrect?