I've been going back and forth between Linux and MacOS a lot and I 
finally noticed that the time was all screwed up in Linux. So  I fixed 
the problem by setting the system clock from a time server and then 
doing:
hwclock --localtime --systohwclock

Now the clock seems to know it's running in local time. I'm still 
curious, however, about the whole notion of system time being separate 
from the hardware clock time. AFAICT, Linux is the only OS that makes 
the distinction. I can't quite understand what the distinction is for.