getDisplayText cannot return value of date field

[Frank Ho]

Hi,All
    I placed a date field in a cell, and choosed a date.
    In code, I got this cell via getCellByPosition(), then use getDisplayText, 
but it returned an empty string.
    While if I placed a ListBox in this cell, then use getDisplayText, it can 
return the value correctly.
    How can I get the value of date field in a cell?
    Thanks!