I'm not an expert but if ADS-B does not have a native timestamp what is the point of including positional information if there is no time associated with it? why would that be done?
Normally both the transmitter and receiver are synchronized to UTC, so there is no need to send the time in the actual message.
Assuming the transmitter is coupled to UTC, it will send ADS-B messages in 0.2 second time slots. E.g, at 17:00:00.0, 00.2, 00.4, etc. The position sent will be
interpolated to match these 200ms slots. E.g., at 00.35, the transmitter will calculate where the plane will be at 00.4, and then actually send the message out at 00.4. The receiver will do the reverse and determine the correct position and time to 200ms resolution.
If the transmitter is not coupled to UTC, the receiver accepts the position at the receiver's "current time".
There is one bit in the DF17 format that's used to determine if the transmitter is UTC-coupled or not.