John,
their "time stamp" with a line break is treated as text. That's why it wraps in the cell and that's why it results in the calculation error.
I am on a NZ date setting, with the default date format in the order mm/dd/yyyy and when I open the file, your dates display as 30/06/2010 whereas "their" dates show as 6/30/2010, which tells me that they are not real dates, otherwise Excel would have swapped the day and month around.
I don't think you can format a date/time combination to include a line break, but you can create a cell for display purposes only
=TEXT(INT(C3),"mm/dd/yyyy")&CHAR(10)&TEXT(MOD(C3,1),"hh:mm")
You can perform your calculation on the real time stamp data. Hide columns you don't need.
See attached
cheers, teylyn