hardware timestamp issues

I'm writing an input device plugin.
When I use the timestamping mechanism found in FBDeviceOptical (i.e., the fake
counter, see ordeviceoptical_hardware.cxx), everything works fine. But when I
try to generate a hardware timestamp from my device, recording stops working
properly. That is, it allows me to click 'record,' and create a new take, etc...
but after I click 'stop' I don't see any keyframes recorded. 
My timestamp is generated on the device and read via tcpip. In motionbuilder I
convert everything to millisecond values, starting from time 0 at the beginning
of the take. 

Not only am I unsure why my scheme is not working; I'm also unsure why the
optical sample DOES work: when I look at the millisecond values in the debugger,
I see 0,11,22,0,11,22 repeating like so. 
What exactly is motionbuilder expecting that I'm not giving it?

Thanks in advance.