getLinearMediaPlayheadEpochMillis

This method is used to provide values for the LINEAR_MEDIAPLAYHEAD_UTC macro. LINEAR_MEDIAPLAYHEAD_UTC is the absolute UTC playback position on a linear feed derived by the host app from the linear feed's ad-break play point. Unlike the standard VAST MEDIAPLAYHEAD, this value carries a full date and is emitted in ISO-8601 YYYY-MM-DDTHH:MM:SS.mmmZ (UTC). The value is provided as epoch milliseconds (UTC); the SDK is responsible for formatting. Applicable to all tracking pixels. Negative values are considered invalid. default value is null