Sorry that the join session / play button didn't work for you -- we know that can be very frustrating.
Generally when we see the join/play button not working for someone there's a few key problems that could be the case, so hopefully one of the below solutions will work for you.
Day/Time Mix-up -- You'd be surprised but it happens, the upcoming session you are looking at may be for another day/time than you were thinking it was i.e. Tomorrow or AM vs PM -- have a close look if that's the case don't worry you aren't alone -- it wouldn't be the first time we had this come up :)
Time Sync Issues - For whatever reason, your browser isn't recognizing the time correctly -- you can check this by going to http://time.is and looking at the top to see if it corrected your time (see screenshot below)
Depending on your computer, you may need to reset the time on your computer using a time sync feature or change it manually to a different time then changing it back. Here are some instructions on how to sync to a time server, which is offered within Mac and Windows.
Time Zone Issues - Your time zone on your computer isn't correct (less likely if you haven't changed locations or travelled recently but worth confirming) -- check that in the settings on your computer.
Focusmate Settings - Another time zone related issue but this time on Focusmate, maybe your time zone with us isn't set correctly? -- you can edit that here
If you need help with this, please let us know via the help button to the bottom right.