For some reason your function was seeing the "dte" parameter as an integer. Not knowing the entire setup of your model it is difficult to say why.
I can explain this.
All model elements are strings. Even if they are dates or extents or feature classes, they are stored and passed from one tool to another using a string representation, with a data type ("Long", "Date", "Extent") to tell tools what data types they are so you can connect them to parameters. For example, if a tool parameter is an integer the model element's data type is what causes you to only see integer parameters in the pick list that appears when you connect the model element to the tool.
However in the Calculate Value tool you need to explicitly include % signs and quotes so Python sees the element value, and knows it is a string, as Python can't see those "handles" on elements, what is in the variable (string) is simply subtitued between the % symbols. So, in your case 6/15/2019 is being interpreted by Python a Python expression of as two integer divisions, left to right (result zero):
>>> 6/15/2019
0
To have Python see a string you need to use this expression so Python sees a string:
"%dte%"
So, when we have feature class variables we pass to Calculate Value, it's important to use raw strings so the back slashes aren't interpreted as escape characters:
"%input feature class%"
r"%input feature class%"
Hope this is helpful!