The COUNT function in SharePoint Online is used to count the number of arguments that contain numbers in the fields you put within. It cannot be used to sum all values in this field.
In addition, calculated column formulas can only be used to do horizontal calculations for the current row. It cannot do vertical calculation or sum across items.
As a workaround, you can build a flow in this list and sum all Date left values, then do the rest of the calculation and update the column value for you.
Here is a related tutorial for your reference: Calculate Sums and Show Totals.
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