Since most Garmin handheld GPS units are set to record at a maximum of 1 point per second, I don't think there is a way to do that, although their internal clock is much more precise. Some Garmin handheld GPS units do have a stopwatch function and that may help you some, well at least they go to .01 second precision, FWIW...
I need 0.1s accuracy for getting astronomical observations.
I doubt you can get that from an eTrex Legend Cx but there's the GPS 18x 5Hz although that only provides a 0.2s output.
Edit: According to the GPS 18x 5Hz manual
"The GPS 18x-5Hz outputs a UTC with a tenth-of-a-second precision: 123456.8, for example."
Edit 2: FYI the NMEA standard is the mechanism by which you'll get information from a GPS unit. This is typically via a serial port (I don't think the eTrex Legend Cx has one) although it's can also be sent via USB (not sure Garmin implements that). Does your astronomical equipment accept NMEA input?
