I was playing a single hardcore world and died at night. I went to the main menu, deleted the hardcore world and created a normal one (this was done in less than 5 seconds). During the new world load I saw the old world, and a skeleton, that killed me.

And in the new world, I got time glitch.

Time switched from day to night and back once a second.

I closed Minecraft and loaded the new world again. This time it worked perfectly normal.

Attached are screenshots, made by me in a second, new world and remains of an old one.