Fundemental problem
And assume that all of my testing is being performed in a desert area of the American southwest.
There is a fundamental problem that most time travel designers oversee. They all seem to assume that planet earth's position is at the center of the universe.
You are traveling through space-time. Let's say you assume to keep your position in space coordinates and you travel back or forward for only a few minutes, you are going to miss earth big time.
Wikipedia:
Earth's orbital speed averages about 30 km/s (108,000 km/h; 67,000 mph), which is fast enough to cover the planet's diameter in 7 minutes and the distance to the Moon in 4 hours.
Taking into account the speed of the solar system itself (Again wikipedia):
The Solar System is traveling at an average speed of 828,000 km/h (230 km/s) or 514,000 mph (143 mi/s) within its trajectory around the galactic center.
Now, this could go on taking all movements and expansion of the universe in account, but I will stop here. Point is: you will need a pretty good coordinate calculation system before you send your time machine. This can be done, if you know exactly what time you are sending it to. But there appears the paradox: you want to send it back in time to calibrate it's "Time dail". It is therefore impossible to put it anywhere on the planet.
Problem becomes the solution
So, don't try to land in earth. Make sure that the design is capable of withstanding outer space. Send it to exactly the same space coordinates, which will put it somewhere in deep space, with the best view of the galaxy you can ever get.
Now, it can do wavelength identification of our own solar system and other known stars or even remote galaxies. 921
And based on the absolute or relative differences, you can accurately calculate the time.
Risk calculation
Before sending back your machine, try to estimate its inaccuracy. Based on the galactic model, try to keep the probability small it ends up in another star that since traveled to the same coordinates. Changes are really small, but the calculation is not difficult vs loosing your precious machine.
More runs
Do more runs, to different time frames. Try do to confirmation measurements that the galactic model is indeed correct. Each run your "Time dail" should become more accurate. Once you have enough confidence, start tuning the coordination system.
Do the same range of trips, but closer to earth. Assess earth's position and own rotational speed. Try to create your own calibrated movement model, not based on calculation of modern science, but what is now actually measurable.
In the final test you send it in the same place in the south-western American desert. If the machine returns in one piece (eg you did not put it in earth's crust or let it plunge down from a reasonable height) your machine is ready for human travel! 234