Inertial observers can legitimately use the famous time-dilation result of special relativity to determine simultaneity at a distance. Observers who are currently accelerating can't. To be an inertial observer during some period of your life, do you have to be a PERPETUALLY inertial observer? I.e., is it required that you must NEVER have accelerated in the past, and that you can guarantee that you will NEVER accelerate in the future? Or, can you be an inertial observer if it has been long enough since you stopped accelerating, and if you can guarantee that you will not accelerate for some period of time into the future? Or, can you be an inertial observer for some period of time, provided that you don't accelerate during that period? The question matters, because the answer specifies WHO is entitled to use the famous time-dilation result, and WHEN can they use it, in order to determine simultaneity at a distance. Different answers to that question have produced several different published procedures for answering the question, "How old is that particular distant person, who is moving with respect to me, RIGHT NOW?". Dolby and Gull, in their "Radar Simultaneity", say that an observer is an inertial observer if he has not accelerated too recently, and will not accelerate too far into the future (and they exactly specify how much is too much). Dolby and Gull's method is clearly non-causal. Minguzzi says that an observer is an inertial observer if he hasn't accelerated too recently, but there is no requirement that he can't accelerate at any time in the future. The "momentarily co-moving inertial frames montage" (MCMIFM) says that an observer is an inertial observer if he isn't CURRENTLY accelerating, even if he has accelerated infinitesimally-recently in the past, or will accelerate infinitesimally-soon in the future ... i.e., he can use the time dilatation result throughout any period of time in which he is not accelerating. What say you?