That’s military terminology for the number of days you have left in a specific place before the day you leave (that’s the wakeup part of it).

I’ve had a struggle with the COPD in that the pulmonary doctors said I should probably take a High Altitude Simulation Test to see whether I could fly in a pressurized cabin. Why should the doctors think I couldn’t? Well,

Based on FAA flight regulations, aircraft cabins are pressurized to between 8,000 and 10,000 feet, which means, compared to conditions at sea level, the amount of breathable air (including oxygen) is reduced by about 6%.

So the medical profession devised a test which would simulate the air that you get on the plane, and they measure the amount of oxygen in your blood.

[The test] uses a mask and a valve connected to an H-cylinder tank, which supplies the patient with a low FiO₂ mix of 15% oxygen on inspiration. The patient also wears a nasal cannula connected to a pulse oxygen tank regulator that the technicians use to titrate additional oxygen, based on the patient’s oxygen saturation levels, which must stay at or above 90%…

They put an oximeter on my index finger, fit me with a mask and fed me the simulated cabin air. Poof! The oxygen saturation level in my blood immediately dropped from 91% to 87%. The nurse said “okay, you’re gonna need oxygen.” She fit a cannula into my nose and turned the oxygen on at a pace of 3 liters per minute. That bumps your oxygen level up from the 21% level found in ordinary air to roughly 32%, a richer mixture.

Okay. So now I know I have to lug a portable oxygen concentrator on the plane (you can’t carry tanks). That’s a damned nuisance, but I can live with it. But getting the authorization from the VA and then getting it to send the authorization over to the local medical supplier has taken a week and raised my stress levels considerably. Friday, nine days after my doctor requested it, the supplier finally got the necessary documents. I still need signatures on several forms from my doctor, I can’t pick up the oxygen device till Monday and I don’t know how big and heavy it might be, I have to notify the airline of the gadget’s manufacturer to be sure it fits United’s requirements (which include a 14-hour battery life based on the number of flight and layover hours on my schedule) and I’m supposed to get on a plane Tuesday evening.

Arrrrgh!