Sounds interesting and would really like to help - please attach your twbx workbook so we can see how far you got and where you got stuck
I use quite sensitive data so I decided to show similar example using Superstore dataset.
There is a dashboard with Calendar heatmap for a particular period (one month) and Overdue worksheet where I want to show the number of Orders with Date earlier than chosen Date on a Calendar Heat Map. Currently it's Today() but obviously this dataset contains Order Dates earlier than 2019.
Superstore example.twbx 460.0 KB