How do I enable day/night cycle in old Minecraft world?

I have been trying to figure this out for days now. A world of mine that I made a very long time ago in MCPE (back when glass, bricks and shears were unlimited and worlds were limited to 256 blocks in size–can’t remember which version I created the world in) somehow got frozen in time. I’m trying to fix that without enabling cheats so that I can still earn achievements in the world. I tried NBTedit and NBTExplorer on the level.dat file, but both have failed. Nothing else I’ve tried has worked either. Please help.