Clark Moody's dashboard estimates April 21. I'm interested in the formulation used there.

For Nodeyez it's a very simple estimate taking the total time that has transpired during the current halving, divided by blocks to get average block rate and then project forward. A possible improvement could be plotting per difficulty epoch and identifying a curve for where it may meet for the halving. But still, with less than 10 months to go, a 5 day difference between Nodeyez and Clark Moody is quite massive