I'm glad it worked for you. Sorry hasn't fixed my problem. Maybe different. I can tap a date sometimes and get a black dot, like I always did on iphone. Other times, I can't get a black dot on the date. It's not a change in functionality. It simply works sometimes and not other times. To get it to work, I must page forward months ahead and then go back to the month that wasn't showing the black dot. Worse, I bought two of SE phone on the same date in September 2016. One has the problem, the other doesn't. That sort of problem doesn't sound like software to me. I have most up to date OS. I smell hardware. Its almost like the memory is running out.
Amazing. All I want is to highlight a date with a black dot so I can change things and it simply doesn't work consistently. Too big a problem to be lightly called a "bug" in something so fundamental as a calendar. I posted here in case there are others like me.