SSIS Derived Column Transformation Question

I have a derived column (ParsedStartTime is nvarchar) created by the following expression:

SUBSTRING(Time,1,FINDSTRING(Time,"-",3) - 1)

I also have an input field (Setup T is Float)

I want to create a derived column (CalculatedStartTime) that is ParsedStartTime - Setup T.

I would like to do this in the same table.  I cannot use ParsedStartTime as input (because it is derived), but can I create an expression using the same code and then DATEADD to reduce the time by Setup T?  Any idea what the expression would look like?