The timestamps in Muse Monitor come from the Interaxon data packet timestamp.
Regarding accuracy and sync, it should be noted that the timestamps do not come from the hardware itself, but are created by the Interaxon API during processing of the Bluetooth data, using the device system time.
As the Bluetooth data stream is buffering and processing packets in chunks, this results in timestamps which have a large delta from the expected 256Hz rate. You will see many packets which are processed very quickly, then a few which have a large delay (relatively speaking, on the microsecond level).
On the hardware side, packets are in fact generated at a steady 256Hz rate, so when processing for sync, I would advise averaging out the timestamp deltas.
Regarding UTC, yes that's something I could add as an option. I will look into it.