Determining conversion rates for on-line purchases

A computer-implemented system for identifying on-line advertising conversion, the system comprising:

• an advertising sub-system arranged to serve interactive advertisements to users of computing devices;
• one or more transaction logs that store transaction instance data representing interactions by computer users with the advertisements and with products that are subjects of the advertisements;
• and
• a conversion correlator programmed to correlate connections between interactions with advertisements with interactions with products that are subjects of the advertisements by matching clock skew data for the computing devices.

A computer-implemented method comprising:

• receiving, at a computing device, input that indicates that a user has selected an advertisement being displayed by the computing device;
• in response to receiving the input, determining a first clock skew for the computing device relative to a computer server system at a first time and providing the first clock skew;
• and
• executing, by the computing device, code associated with content that is related to the advertisement and that was purchased by a user of the computing device, wherein execution of the code causes the computing device to determine a second clock skew for the computing device relative to the computer server system at a second time and to provide the second clock skew;
• wherein the first clock skew and the second clock skew are used to verify that selection of the advertisement and purchase of the content was performed by the computing device.