A system comprising: a data processing apparatus; and a computer storage medium encoded with a computer program, the program comprising instructions that when executed by the data processing apparatus cause the data processing apparatus to perform operations comprising: receiving a digital video comprising video data; processing the video data to detect first features indicative of a human face in the digital video; determining, from the video data, a first frame, in which the first features indicative of the human face are detected in the digital video, marking when a person corresponding to the human face appears in or enters the video; determining second features of the person corresponding to the human face; determining first timestamp data corresponding to the first frame; processing at least one second frame from the video data and for each second frame, determining that the person continues to be present if the human face is also detected and if the human face is not detected, processing the second frame further and determining that the person continues to be present if one or more of the second features are detected; determining second timestamp data corresponding to the at least one second frame, marking a last appearance or exit of the person from the digital video; generating an identifier corresponding to the human face; generating a data set comprising the identifier, the first timestamp data, and the second timestamp data; and appending the data set to the video data to provide annotated video data; wherein the annotated video data indicates a duration of the digital video in which the person corresponding to the human face is deemed to be present, wherein the duration of the digital video is defined by a difference between the second timestamp data and the first timestamp data. a data processing apparatus; and a computer storage medium encoded with a computer program, the program comprising instructions that when executed by the data processing apparatus cause the data processing apparatus to perform operations comprising: receiving a digital video comprising video data; processing the video data to detect first features indicative of a human face in the digital video; determining, from the video data, a first frame, in which the first features indicative of the human face are detected in the digital video, marking when a person corresponding to the human face appears in or enters the video; determining second features of the person corresponding to the human face; determining first timestamp data corresponding to the first frame; processing at least one second frame from the video data and for each second frame, determining that the person continues to be present if the human face is also detected and if the human face is not detected, processing the second frame further and determining that the person continues to be present if one or more of the second features are detected; determining second timestamp data corresponding to the at least one second frame, marking a last appearance or exit of the person from the digital video; generating an identifier corresponding to the human face; generating a data set comprising the identifier, the first timestamp data, and the second timestamp data; and appending the data set to the video data to provide annotated video data; wherein the annotated video data indicates a duration of the digital video in which the person corresponding to the human face is deemed to be present, wherein the duration of the digital video is defined by a difference between the second timestamp data and the first timestamp data. receiving a digital video comprising video data; processing the video data to detect first features indicative of a human face in the digital video; determining, from the video data, a first frame, in which the first features indicative of the human face are detected in the digital video, marking when a person corresponding to the human face appears in or enters the video; determining second features of the person corresponding to the human face; determining first timestamp data corresponding to the first frame; processing at least one second frame from the video data and for each second frame, determining that the person continues to be present if the human face is also detected and if the human face is not detected, processing the second frame further and determining that the person continues to be present if one or more of the second features are detected; determining second timestamp data corresponding to the at least one second frame, marking a last appearance or exit of the person from the digital video; generating an identifier corresponding to the human face; generating a data set comprising the identifier, the first timestamp data, and the second timestamp data; and appending the data set to the video data to provide annotated video data; wherein the annotated video data indicates a duration of the digital video in which the person corresponding to the human face is deemed to be present, wherein the duration of the digital video is defined by a difference between the second timestamp data and the first timestamp data.