OffsetTimeMs sets the optional parameter "offsetTimeMs": The offsetTimeMs parameter specifies a positive time offset when the specified slate change will occur. The value is measured in milliseconds from the beginning of the broadcast's monitor stream, which is the time that the testing phase for the broadcast began. Even though it is specified in milliseconds, the value is actually an approximation, and YouTube completes the requested action as closely as possible to that time.
If you do not specify a value for this parameter, then YouTube performs the action as soon as possible. See the Getting started guide for more details.
Important: You should only specify a value for this parameter if your broadcast stream is delayed.