Thanks, Mark. This shows more variance than I had read about, and observed.
Your device, or the software you loaded it into on your computer, will have a setting. Normally in Setup, or Setting. normally called Position Datum, or Coordinate Datum, from what I have seen.
Also, for ease of entry, the Coordinate Format is simplest to enter if it is Decimal Degrees, or DD.ddddd. Also a setting on your device, or in the software.
Keep in mind, though, that once you have manually entered coordinates into your device, if you change the datum, it will translate the location to the other datum. It won't keep the numbers you entered, and reposition you in the other datum. You will have to change datums, then re-enter a waypoint location.