t.column "start_at", :time
This works on my local machine, when I move it
out to RailsPlayground I am getting this error.
I'd recommend you put together a very, very minimal sandbox app and try that on RailsPlayground. By minimal I mean, for example, take out everything in the table except start_at. Have one controller with two methods, one to display the view for entry of the date, and one to save the result to the database. Have two views, one that allows the entry of that field, if that's what your app allows and the other that displays the value that got saved or an appropriate error message. If the problem is still demonstrable, then document the config of your local machine including gem list, plugins, versions of everything, and submit a ticket to your support team. In my experience, you definitely need to have something the support team can work with to get them engaged.
Best of luck,