How about nanoHz?
Here are the readings from my counters.
Racal Dana 1991/1992 measures 1 Hz as
1000.XXXXXXe-3 with a 1 second gate time (measurement takes 2 seconds) or
1000.XXXXXXXe-3 with a 10 second gate time
Agilent 53131A measures 1 Hz as
1.XXX,XXX,XXX,X with a 1 second gate time (measurement takes 5 seconds) or
1.XXX,XXX,XXX,XX with a 10 second gate time
Fluke/Philips PM6681 measures 1 Hz as
1.XXXXXXXXX with a 1 second gate time (measurement takes 5 seconds) or
1.XXXXXXXXX with a 10 second gate time (i.e. same reading)
Some counters measure period faster and/or more accurately than frequency. Odd, but true.
Ed