I think I may have found a bug in the Hub Variable creation process.
When I attempt to create my 63rd DateTime Hub variable, the system throws an error.
If I then click on Settings -> Hub Variables, sometimes it throws the same error for a few seconds. It will then eventually show me the list of variables, and the DateTime variable I tried to create shows up in the list, but only with a time:
I have deleted this variable and added a new one many times, trying to change the name, change the time, change the date etc., but the same thing happens.
I also restored the whole database once, which deleted ~13 DateTime variables I had recently added. Then it lets me add back a lot of the DateTime variables until I try to add the 63rd one, and the same error cycle happens. If I ignore this error and keep the erroneous variable in the list and try to add another DateTime variable, the same error happens on that additional DateTime varibale.
For reference I have 136 total variables before trying to add this 63rd DateTime variable. If I try to add another variable type, for example a Number, that is allowed without error, even if I leave the erroneous DateTime variable in the list.
So it doesn't seem to be a problem with the total variable count exceeding some limit, but perhaps is an issue with the quantity of DateTime variables I am trying to create.