I just noticed this on Friday. All day Friday, it showed the current day as Thursday. Yesterday it showed the current day as Friday. Today it’s showing Saturday.
I did a close-and-reopen on y phone, which usually fixes software glitches, but it’s still doing it.
It’s getting the weather right for the day, just has the day wrong.
Do you mean you just closed and opened the app? Have you done a shutoff and restart of the whole phone? Try that first. If that doesn’t work, try uninstalling and reinstalling the app.
Only thing I can think of is you’re looking at the wrong location. Can you swipe to the left or right revealing another set of weather data, possibly with the same city name (but the ‘wrong’ city’)? Although it could only ever be a day ahead, not a day behind. What version of iOS are you using? I’m at the current iOS 9.x and my Apple Weather app (which I never use, I use Yahoo Weather! instead) doesn’t show “Updated date time” anywhere at the bottom.
Is the rest of the iPhone system showing the correct date? Check Settings, Date & Time, and see if Time Zone has Set Automatically turned on.