I have found I can edit the gpx, by doing a replace all of 1935-12, with 2016-04. Not the best having the unit's date decades out, but could be a work around.
Sorry, I meant replace
2035-12 in the gpx with 2016-04...
HCx Unit time = 1935, gpx time = 2035. Does that change anything?
I did the reset followed swiftly by 'super master reset' again. 1935 remains.
I can try draining the internal battery for a long period. To date only done o/n; though who knows it could take months to discharge...
Someone has looked at the circuit board battery for a HCx satellite issues:
This is a small ~6mm diameter coin cell located near the ribbon cable connector for the micro SD card. I'm not sure if this cell is rechargeable or not. I do know that I measured the cell's voltage at about 0.25V. I desoldered the cell, and briefly applied 3V to the terminals to charge it up past 2V. Once I soldered the cell back in, the unit immediately acquired satellites upon rebooting.
This is interesting, from t'interweb:
I had this problem when I first got my legend. The problem is (apparently) that when the almanac was downloaded from the satellites it got corrupted, and that causes a date offset. The almanac won't expire for something like 6 months, so until then, you will have the problem. What you need to do is a master reset on the unit, and that will force it to download the almanac again. However, you will lose all stored waypoints, tracks, routes, etc. (Strangely, you don't lose loaded maps).
Update - Garmin support option hit the buffers. Corrupted almanac/time can't be reset it seems, and they no longer take in units for repair (which I wouldn't do anyway).
So it's wait for the battery to discharge, or use & edit the gpx file.
edit.Interestingly Garmin says
'there is no internal battery in the eTrex Vista HCx. ... It relies solely on the AA batteries inserted'. That's it with this unit I guess (so edit 2035 in gpx to 2016).
In the course of reseting, I discovered that the test data screen can be made to change to multi coloured rectangular frames moving out towards the edge of the display. Funky.