Command blocks don't use @p, @a, or @r correctly

When I was trying to make a time machine with a command block set to "/tp @p -560 56 -343", the command block tried to teleport the player "@p" instead of the closest player. So I just had to put my playername in just to test. Before I reported this, I watched a few YouTube videos on the snapshot, and they all said that "@p doesn't work" or "I have to put my name in, I forgot that @p didn't work".