Display the date and time each time the data in a cell range is changed

I'm not familiar with macro's, but found one which accomplishes to change the date and time each time a cell in a column range is changed.

UDF:

Public Function UDF_Date(ByVal data) As Date
UDF_Date = Now()
End Function

Formula above range:
=UDF_Date(H7:H25)

Example:
H4 23/08/2012 09:00

H7 12
H8 40
...
...
H25 15

If any entry is made in cells H7 - H25, the date and time should be logged in cell H4.

However, when I open the excel sheet and enable macros it automatically changes the date and time to the date time the sheet is accessed. This also happens when I send the sheet via e-mail and the person on the other side opens it.

How can I correct this and embed the date with it only changing if one of the cells in the range changes?

Thank you