Most modern Internet-connected PCs should be synched via NTP;
Most Internet-connected PCs are synchronized to time.windows.com, which
is not NTP.
Windows Time Service is "guaranteed" to only within five minute accuracy, although in practice it's usually accurate to within one minute on relatively modern PCs with reliable network connections.
I know ADS-C has a timestamp, but I think you're right that ADS-B doesn't.
ADS-B does have a timestamp:
ADS-B
does not have a message timestamp, so typically the receiver hardware/software will add one or more timestamps.
There is no room in the 1090ES DF17 message format to transmit a full timestamp. More precise timing & location info requires both the transmitter and receiver to independently synchronize to UTC. There is a 1-bit flag in the DF17 format to indicate whether or not the transmitter is coupled to UTC.