For Legacy and Rendering Log messages, this field MUST specify how much of the content has been rendered (played) to the end user, specified in seconds. Time spent buffering data MUST NOT be included in this value.
Playback at non-normal play speed does not affect the amount of content rendered, when expressed in time units. For example, if the client was rewinding the content, the x-duration value can be computed as the absolute value of the difference between the starting presentation time and ending presentation time.
For Streaming Log messages, the x-duration field MUST specify the time it took to receive the content, in seconds.
Fractional time amounts MUST be rounded to the nearest larger integer value.
The value MUST be an integer in the range from 0 through 4,294,967,295.
The syntax of the x-duration field is defined as follows.