Wrong DataType Parsed Into Function

I have a function that starts as follows:

Function Timings(StartTime As Double, EndTime As Double, Times As Range, Calc As Long)

StartTime will refer to a cell in Column A that holds a date and time EndTime will refer to a cell in Column B that holds a date and time Times is the range of cells that hold times of the day I am testing against. The cells are within a range defined called Hours. Calc refers to different criteria that I am testing again. This is a number list between 1 to 5.

The function works well if the cell that StartTime or EndTime refer to cell values that are date/time. If the data is anything different, I get a #VALUE error such as if the cell contents is text. I tried to trap the error to see if the data inputted in the cell is not date/time and thus cannot be converted to a Double but it always throughs up #VALUE and the function seems as though it fails when it . Is there anyway to trap the error of wrong data type in the first the first few lines of code in the function?

Example

Column A Column B Column C Formula
05/01/2013 12:00 06/01/2013 09:00 =Timings(A1,B1,Hours, 1)
25/02/2013 15:00 30/02/2013 14:30 =Timings(A2,B2,Hours, 1)
03/01/2013 16:00 abc =Timings(A3,B3,Hours, 1)

Row 2 will return #VALUE becuase 30/02/2013 14:30 is an invalid date (30th Feb does not exist so Excel seems to pickup as text)

Row 3 will return #VALUE because it is text and cannot be converted to a double data type