There are many reminder scripts out there, but this is mine.
This is a quick little script I wrote to remind me of things. The current version is macOS-specific in that it uses say and osascript to both speak the reminder as well as display it in the Notification Area.
I included an empty section for Linux for when I am sitting in front of my Linux box and want the same functionality; certainly it's a little bit trickier insofar as the myriad of notification systems available.
Hope this is useful for you. If you do make it work under Linux, PRs are always welcome!
I put the script in $HOME/.local/bin; it can go anywhere you have permissions for and is on the path.