Technically, GUMO is still in the Central Time Zone; however, the park unofficially observes Mountain Time, probably because that puts it in the same time zone as Carlsbad Caverns National Park and the administrative office for both in Carlsbad.
Maybe. Who knows? Some years ago I saw a detailed map of where the time zone boundary ran in the vicinity of the park, but cannot now find it. That map implied a legal change to the boundary location, but I also cannot locate any references that such has officially occurred.
However, that map had an interesting layout as the line was immediately east of US 62/180 and turned west (running north of 62/180) in the flats below Guad peak until intersecting the Hudspeth county line, where it turned south. While not marked on the highway, if driving to El Paso you briefly left the MTZ and reentered the CTZ.
Regarding time zones, as I have noted in the past, it makes zero sense for Big Bend to be in the Central time zone. No apparent political or economic interest is served by staying in the CTZ and no detrimental effects would occur if it properly were placed in the MTZ, which is logically where it belongs. Moving the line would eliminate sunrise and sunset being quite obviously out of sync with the latitude (as long as we continue to play the time zone game...see below).
This is an endless source of confusion to park visitors, whose cell phones often hit towers running on Central Time and thus think the time is one hour later than what is observed in the park.
A genuine first-world problem. Before mobile phones, no one paid much attention to this "problem" and life went on. You either knew where you were and what time it was, or you didn't, and you either were on time for things or not.
If you want to be totally confused, and bang the hell out of your mobile phone swapping times, take a particular drive through NE Arizona.
This route carefully was selected to maximize the number of changes due to a peculiarity of the Hopi/Navajo boundaries. With respect to this situation, the Hopi reservation is completely surrounded by the Navajo rez; however, a tiny piece of Navajo is completely surrounded by Hopi and, while shown on detailed maps, is a factoid oblivious to many.
Arizona and the Hopi do not observe DST
The Navajo and Utah use DST
Roads are AZ 77, AZ 264, US 160 and US 87
Start in Holbrook AZ on I-40 and go north on AZ 77 onto the Navajo reservation, into the Hopi Pueblo, into the Navajo portion surrounded by Hopi, back onto Hopi, across a short piece of Navajo, back onto a Hopi outlier, into Navajo for the last time, off the rez into Page and cross into Utah northwest of Page on US 87.
Using 10 am (and assuming you do all this within an hour so as to make it simple), here is what you will find as a result of that damned daylight savings time:
Holbrook, 10 am
Navajo rez, 11 am
Hopi rez, 10 am
Navajo, 11 am
Hopi, 10 am
Navajo, 11 am
Hopi, 10 am
Navajo, 11 am
Page, 10 am
Utah, 11 am
A mere nine! (9) time changes in a distance of 235 miles. Does anyone do these changes? Of course not. In fact, some of it doesn't even register on mobile phones, but it all exists.
Generally, it affects no one other than the locals, who are well-versed in what time it is and when openings/closings occur.
This extreme example illustrates how idiotic and unnecessary is DST; it should be legislated out of existence, as Arizona and Hawaii very wisely have done on their own. However, there are politicians (I believe Rubio is one) who want to make DST a permanent year round event. As comedian Chris Rock intoned in his video "How To Not Get Your Ass Kicked By The Police" the only phrase that comes to mind is "that's just ignorant."
Actually, there long has been two proposals which would solve a lot of problems, but are unlikely to ever happen.
The first is to put the US on only two time zones, with no DST: Eastern and Western. The line would be where the current CTZ/MTZ line is (that proposal does not change the idiocy of the west TX situation. One variant has the zones separated by one hour; another has a two hour separation. Either would be an improvement.
The other, far more logical solution, is to put the entire planet on Universal Time (UTC), with no time zones. Undoubtedly, those that go to work at 8am and then find themselves going to work at 5pm would be resistant due to an inability to accept change. Actually, UTC runs on a 24 hr clock, so the problem of which o'clock it is is forever eliminated, and you would go to work at 1700 hrs instead of 0800.
UTC (erroneously and frequently called 'military' or 'scientific' time) is just a much simpler system. Folks can get used to anything (unless they are Luddites...as we are with the metric system in the US). Aviation, communications, international interests, NIST clocks and a myriad (or perhaps a plethora) of others run on UTC even if you don't see it. UTC makes the most sense and eliminates the issue of what time it is anywhere. Yeah, the sun might rise at 1500 hrs in Big Bend, but once you stop having to change clocks due to latitude or DST, everything becomes so much simpler. Even the haters would learn to like in short order.
Even BBC is on 24 hr time. Check out the post times for the last half of the day.