Thanks for getting back to me over this. Just wanted to provide a clarification regarding the previous post. I was able to use the second snippet to stack the labels and avoid zero values. However the date string is only appended to the first attribute field value in the iteration. Here is a screenshot of the results :
I tested this in ArcMap 10.6 as well as ArcGIS Pro 2.1.2 and received the same result. The text string is not appended to every attribute value. Only the first value is appended through the iteration.
However I was able to find a script that does provide required results;
Using a for loop in a list comprehension does append the text string to each attribute value in the iteration. Here is a screenshot of the results.