You may or may not be able to set a Date type to null. I think you'll probably have to convert to string if you want to do that.
So, in your main report, you'll have a formula like this that will initialize the date:
Shared StringVar myDate := ""
Place this formula in a a section above where the subreport is located.
Change the formula in the subreport to something like this:
WhilePrintingRecords;
Shared StringVar myDate := ToText({Schedule.App_DtTm}, "MM/dd/yyyy")
Note that I left the semi-colon off the end of this - in order to get the value out, you should do that on the last line of any formula that returns a variable.
Then, back in the main report you'll use something like this to display the value:
WhilePrintingRecords;
Shared StringVar myDate
-Dell