Serious as a heart attack. Try to figure out whether you'll make it to Meteor Crater before closing time from Blanding UT during DST. You call them up, they're open till 6:00. Arizona Time (Mountain Standard time all year round.). You're charging in Blanding - Mountain Daylight time, so the car is an hour ahead. You drive into Arizona, the car switches to AZ time as soon as it acquires an Arizona cell tower. Then you enter the Navajo nation, which honors DST, and the car flips back. Then you go thru Hopi territory (an enclave inside Navajo country) which follows AZ time, it switches again. Then out of Hopi, back to Navajo. Then out of Navajo into Arizona proper. You don't know what the real time is for like 9 hours running. "Do we need to charge? Probably. Do we have time to charge? How should I know?" We charged in Bluff at a J1772 during lunch, but decided we didn't have time to hit the Flagstaff SC too. Ended up doing an hour at an RV park just outside of Meteor Crater to make it back to Flagstaff.
It doesn't help at all that my wristwatch is time-synched to my phone, which also does the time-zone dance along with the car. I guess I could lock in on MST when I'm traveling to have a known reference.
But on any road trip, if you're calling into a Hotel to make reservations and the Hotel is in a different time zone (or you're telling your relatives you'll be on-time for dinner), your ETA local time might be off by an hour. This, of course, implies that the ETA in the nav block should definitely be calculated (and marked) in the local time zone, so it doesn't abruptly change when you cross the line.
It might also be useful for the lines between Time Zones to be prominently displayed on the map. With a big red X over Arizona. Of course if you talk to an Arizonite, he/she will tell you they're patiently waiting for the whole rest of the country to get with the time-sanity program.
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