mikeburg,
the SUM() function will disregard text. It will not distinguish between
dates and numbers because dates are internally stored as numbers. Thus,
if you only want the sum of purely numeric values you will need to use
VBA and a helper column.

Function IsItANumber(x)
IsItANumber = isnumeric(x)
End Function

To define this function, Alt+F11 to go to the VBA editor, Insert|Module
and paste the function code above.

Back in Excel, in another column (say E3:E43), use =IsItANumber(D3) and
copy down to row 43. You can then use the following formula:
=SUMIF(E3:E43, TRUE, D3:D43)

HTH
Kostis Vezerides