Timestamp recorded stream

I’d like to record raw broadcast (rx_cfile) with either a USRP1 or USRP2
(have both) but with all samples known to as accurate as the system
clock, preferably millisecond to sub-millisecond accuracy. I considered
writing a block that outputs the number of milliseconds since midnight
as shorts that get interleaved into the datastream?

Has anyone accomplished anything like this before?