How to tell doneness of a boiled egg besides time
Try an egg timer that measures temperature.

Here's an example (that I don't necessarily endorse): Egg Perfect Timer

The idea is, the device supposedly absorbs heat similarly to an egg, and gives a visual signal of the level of heat absorption. If the water is colder than typical, because you're at a higher altitude, the time it takes to heat the eggs and egg timer will lengthen, but the amount of heat the eggs and egg timer need remains consistent.

I had one very similar (perhaps that same brand) when I was much younger.  I remember it being hard to read, but I think it was accurate.