I figured it out after I visited How to convert any date format to yyyy-MM-dd and clicked the link to String Format for DateTime [C#].
I changed:
column_L.NumberFormat = "yyyy-mm-dd";
to
String.Format("yyyy-mm-dd", column_L);
If anyone needs more details, the YouTube video I mention with my original question is: Learn SSIS: 156 How to change excel column to numeric in ssis.
Thanks!